Data processing system having centralized nonexistent memory address detection

ABSTRACT

In a data processing system which includes a central processing unit (CPU) having one or more common buses to which one or more main memory units for storing program software instructions and program data are connected, logic is provided within the CPU for detecting an attempt to access a main memory location not contained in the one or more main memory units present in the data processing system. Logic is provided for detecting the attempt to access the nonexistent memory location for the case where the access was being done in the course of the CPU executing a software instruction or for the case of where the access was being done to transfer data between the main memory and an input/output controller connected to one of the one or more common buses.

CROSS-REFERENCE TO RELATED APPLICATIONS

The following patent applications which are assigned to the sameassignee as the instant application have been filed on an even date withthe instant application, have related subject matter. Certain portionsof the system and processes herein disclosed are not our invention, butare the invention of the below named inventors as defined by the claimsin the following patent applications:

    ______________________________________                                                                       SERIAL                                         TITLE          INVENTORS       NO.                                            Data Processing System                                                                       Jian-Kuo Shen   008,121                                        Having Centralized                                                                           John J. Bradley                                                Data Alignment for                                                                           Richard L. King                                                I/O Controllers                                                                              Robert C. Miller                                                              Ming T. Miu                                                                   Theodore R. Staplin, Jr.                                       Data Processing System                                                                       Theodore R. Staplin, Jr.                                                                      008,122                                        Having Synchronous Bus                                                                       John J. Bradley                                                Wait/Retry Cycle                                                                             Richard L. King                                                               Robert C. Miller                                                              Ming T. Miu                                                                   Jian-Kuo Shen                                                  Data Processing System                                                                       John J. Bradley 008,004                                        Having Multiple Common                                                                       Ming T. Miu                                                    Buses          Jian-Kuo Shen                                                  Data Processing Sys-                                                                         Ming T. Miu     008,005                                        tem Having Hardware                                                                          John J. Bradley                                                Interrupt Apparatus                                                                          Jian-Kuo Shen                                                  Data Processing Sys-                                                                         John J. Bradley 008,003                                        tem Having Data Multi-                                                                       Robert C. Miller                                               plex Control Apparatus                                                                       Ming T. Miu                                                                   Jian-Kuo Shen                                                                 Theodore R. Staplin, Jr.                                       Data Processing Sys-                                                                         Robert C. Miller                                                                              008,002                                        tem Having Data Multi-                                                                       John J. Bradley                                                plex Control Bus                                                                             Richard L. King                                                Cycle          Ming T. Miu                                                                   Jian-Kuo Shen                                                                 Theodore R. Staplin, Jr.                                       Data Processing Sys-                                                                         John J. Bradley 008,001                                        tem Having Direct                                                                            Thomas O. Holtey                                               Memory Access Bus                                                                            Robert C. Miller                                               Cycle          Ming T. Miu                                                                   Jian-Kuo Shen                                                                 Theodore R. Staplin, Jr.                                       Data Processing Sys-                                                                         Ming T. Miu     008,123                                        tem Having Centralized                                                                       John J. Bradley                                                Bus Priority Resolu-                                                                         Jian-Kuo Shen                                                  tion                                                                          ______________________________________                                    

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to data processing systems and morespecifically to the interchange of data between a main memory unit andthe central processor unit or input/output controllers in such dataprocessing systems.

2. Discussion of the Prior Art

A data processing system usually includes a central processing unit(CPU) which executes software instructions which are stored ataddresses, or locations, in main memory. These software instructions aretransferred to the CPU sequentially under the control of a programcounter. The data that is processed is transferred into and out of thesystem by way of input/output devices, or peripheral devices, such asteletypewriters, magnetic disks, magnetic tapes or line printers.Usually the data is temporarily stored in the main memory before orafter the processing by the central processing unit.

In a system having a plurality of devices coupled over a common bus, anorderly system must be provided by which bidirectional transfer ofinformation may be provided between the main memory and such otherdevices. In particular, apparatus must be provided to detect an attemptby a device connected to the common bus to access a main memory locationnot present in the system. This problem becomes more complicated whenmain memory is contained in more than one main memory unit. The problemis further complicated if the system has more than one common bus towhich any of the main memory accessing devices can be connected.

Various methods and apparatus are known in the prior art for detectingattempts to access a nonexistent main memory location. One suchstructural scheme is shown in U.S. Pat. No. 4,001,790 issued to GeorgeJ. Barlow, entitled "Modular Addressable Units Coupled in A DataProcessing System Over A Common Bus" (hereinafter referred to asBarlow).

Barlow discloses a system in which one or more main memory units areconnected to a common asynchronous communications bus along with the CPUand one or more input/output controllers to which peripheral devices areconnected. In this particular scheme, each main memory unit contains aconfiguration switch that is set when the system is installed to theaddress of the lowest location contained in the particular main memoryunit. Further, logic is provided to indicate the amount of memory in theparticular main memory unit thus allowing each main memory unit todetermine the highest address of a location in the particular mainmemory unit. When another device, CPU or peripheral device I/Ocontroller, on the common bus wants to access a main memory location,the accessing device places the address of the main memory location onthe common bus along with an indicator that the request is a memory reador memory write request. Each main memory unit examines the address andif the address falls within the range of locations contained in the mainmemory unit that particular main memory unit gives a positiveacknowledgment on the common bus to the memory requesting device andthen proceeds to access the addressed main memory location and performthe indicated memory read or write operation.

During all attempted main memory accesses, the CPU monitors the commonbus to see if one of the main memory units gives a positionacknowledgment to the memory requesting device and if no unit gives apositive acknowledgment within a preset time period, the CPU generates anegative acknowledgment which is placed on the common bus. In thisscheme the CPU monitors the common bus even for those memory requestscoming from I/O controllers so that the CPU can generate the negativeacknowledgment if no main memory unit gives a positive acknowledgment.

Barlow further provides that main memory parity errors are signaled tothe memory requesting device by common bus signals that are separatefrom those used to signal a positive or negative acknowledgment to amemory request. Each device on the common bus that accesses main memoryhas logic for receiving the positive acknowledgment, negativeacknowledgment and parity error signals from the common bus so thatappropriate status errors can be stored for examination by the softwareprogram.

The Barlow and similar schemes have the disadvantage that each mainmemory unit must have logic to generate a positive acknowledgment if theaddressed location is within the particular main memory unit. Further,system response time is slowed up, and therefore data rates reduced,because the CPU monitor must be set to wait for the worst case responsetime of any main memory unit's positive acknowledgement before it cangenerate the negative acknowledgment. Further, logic must be provided ineach device on the common bus that accesses main memory to receive apositive acknowledgment, a negative acknowledgment and memory parityerror signals. A further disadvantage of this scheme is that: if thesethree signals are placed on the common bus in parallel, the bus must bewider (i.e., contain more lines); or if placed on the common bus inseries, timing logic must be provided in each device and system devicerates will be reduced.

OBJECT OF THE INVENTION

It is accordingly a primary object of the present invention to providean improved apparatus for detecting an attempt by a device connected toa common bus to address a nonexistent main memory location.

SUMMARY OF THE INVENTION

In an embodiment of the invention, a data processing system comprises acentral processing unit (CPU) and a main memory having a plurality ofdata words with each data word having a unique memory address. Thesystem further comprises an input/output controller (IOC) capable ofaddressing the data words stored in main memory. Means are providedwithin the CPU for the detection of an attempt by the IOC to address adata word not physically present within the system and for thegeneration of an error signal to the IOC. Further means are providedwithin the CPU to detect an attempt by the CPU, during the execution ofa software instruction, to address a data word not physically presentwithin the system.

This invention is pointed out with particularity in the appended claims.An understanding of the above and further objects and advantages of thisinvention may be obtained by referring to the following descriptiontaken in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The manner in which the apparatus of the present invention isconstructed and its mode of operation can best be understood in light ofthe following detailed description together with the accompanyingdrawings in which like reference numerals identify like elements in theseveral figures and in which:

FIG. 1 is a general block diagram illustration of the systemconfiguration of the present invention;

FIG. 2 is a block diagram illustration of an example configuration ofthe present invention;

FIG. 3 is a general block diagram of the central processor unit of thepresent invention;

FIG. 4 illustrates the format of the CPU registers of the presentinvention;

FIG. 5 illustrates the word and address formats of the presentinvention;

FIG. 6 illustrates the system bus interface signals of the presentinvention;

FIG. 7 is a general block diagram of the system bus signals shown inFIG. 6;

FIG. 8 is a more detailed block diagram of the CPU shown in FIG. 3;

FIG. 9 is a more detailed block diagram of the control store of the CPUshown in FIG. 8;

FIG. 10 illustrates the CPU scratch pad memory layout of the presentinvention;

FIG. 11 illustrates the place where the CPU registers are maintained inthe CPU of the present invention;

FIG. 12 is a more detailed block diagram of the system bus data controlshown in FIG. 8;

FIG. 13 is a system bus timing diagram of the present invention;

FIG. 14 is a logic diagram of the basic system timing logic of thepresent invention;

FIG. 15 is a timing diagram of the basic system timing logic shown inFIG. 14;

FIG. 16 illustrates a flow chart of the CPU firmware for system startup/initialization sequence of the present invention;

FIG. 17 illustrates the format of the address and data transfers on theaddress/data lines of the system bus of the present invention;

FIG. 18 illustrates the input/output commands encoded on the RDDT linesof the system bus of the present invention;

FIG. 19 illustrates a timing diagram of a memory access sequence on thesystem bus of the present invention;

FIG. 20 illustrates a timing diagram of the CPU command to input/outputcontroller sequence on the system bus of the present invention;

FIG. 21A through FIG. 21D illustrate timing diagrams of the DMC datatransfer sequence on the system bus of the present invention;

FIG. 22 illustrates a timing diagram of the DMA data transfer sequenceon the system bus of the present invention;

FIG. 23 illustrates a timing diagram of the input/output interruptsequence of the system bus of the present invention;

FIG. 24 illustrates the format of the control word transferred on thesystem bus in response to an input/output software instruction of thepresent invention;

FIG. 25 illustrates the input/output function codes of input/outputsoftware instructions of the present invention;

FIG. 26A and FIG. 26B illustrate the formats of the IO and IOH softwareinstructions of the present invention;

FIG. 27A and FIG. 27B illustrate the formats of the IOLD softwareinstructions of the present invention;

FIG. 28 illustrates a flow chart of the CPU firmware which implementsthe IO software instruction shown in FIG. 26A and FIG. 26B;

FIG. 29 illustrates a flow chart of the CPU firmware which implementsthe IOLD software instruction shown in FIG. 27A and FIG. 27B;

FIG. 30 illustrates the interaction between the IOLD softwareinstruction and the program channel table contained in the CPU scratchpad memory of the present invention;

FIG. 31 illustrates the linkage between traps and software interrupts ofthe present invention;

FIG. 32 illustrates the main memory locations dedicated to variousfunctions in the system of the present invention;

FIG. 33 is a general flow chart of the CPU firmware of the presentinvention;

FIG. 34 illustrates a flow chart of the CPU firmware and shows theinteraction between software interrupts, traps, and hardware interruptsof the present invention;

FIG. 35 illustrates the format of the CPU firmware microinstruction wordof the present invention;

FIG. 35A through FIG. 35D illustrate in greater detail the variouscontrol fields of the CPU firmware microinstruction word of the presentinvention;

FIG. 36 illustrates the operations performed by the microprocessor ofthe CPU of the present invention;

FIG. 37A through FIG. 37G illustrate in greater detail the functionsperformed by the subcommands and control field of the CPU firmwaremicroinstruction word shown in FIG. 35C;

FIG. 38 is a block diagram of an input/output controller of the presentinvention;

FIG. 39 illustrates the timing, request and reset logic of an I/Ocontroller shown in FIG. 38;

FIG. 40 illustrates a timing diagram of the timing signals found on thesystem bus and in an I/O controller of the present invention;

FIG. 41 is a block diagram of a main memory module of the presentinvention;

FIG. 42 illustrates the logic of the control store shown in FIG. 9; and

FIG. 43 illustrates the logic of the CPU shown in FIG. 8.

TABLE OF CONTENTS

DESCRIPTION OF THE PREFERRED EMBODIMENT

DESCRIPTION CONVENTIONS

SYSTEM BUS OVERVIEW

CENTRAL PROCESSOR DESCRIPTION

CPU MAJOR COMPONENTS

PROGRAMMING CONSIDERATIONS

Software Visible Registers

Word and Address Formats

Main Memory

CPU AND SYSTEM BUS INTERFACES

SYSTEM BUS A

SYSTEM BUS B

CPU HARDWARE DESCRIPTION

MICROPROCESSOR

SYSTEM BUS CONTROL

CONTROL PANEL

BASIC SYSTEM TIMING

SYSTEM INITIALIZATION

SYSTEM BUS OPERATIONS

MEMORY ACCESS

MEMORY REFRESH

FUNCTION CODE TO I/O CONTROLLER

DMC DATA TRANSFER REQUEST

DMA DATA TRANSFER REQUEST

I/O CONTROLLER INTERRUPT

EXECUTION OF INPUT/OUTPUT INSTRUCTIONS

Channel Numbers

I/O Function Codes

Output Function code Commands

Input Function Code Commands

Software Input/Output Instructions

I/O Instruction

IOLD Instruction

IOH Instruction

Traps and Software Interrupts

Software Interrupts

Traps

FIRMWARE OVERVIEW

GENERAL DESCRIPTION OF FIRMWARE FLOW

SOFTWARE INTERRUPT, TRAP, HARDWARE

INTERRUPT INTERACTION

Software Program

Firmware Microprograms

Software Interrupts, Hardware interrupts and Traps

Software Interrupts

Hardware Interrupts

Traps

Interrupts and Traps

CPU FIRMWARE WORD DESCRIPTION

SCRATCH PAD MEMORY CONTROL

ARITHMETIC LOGIC UNIT CONTROL

SUBCOMMANDS AND CONTROL

READ ONLY STORAGE ADDRESSING

I/O CONTROLLER LOGIC DETAILS

I/O CONTROLLER DEVICE LOGIC

Command Logic

Task and Configuration Logic

Interrupt Logic

Status and Device Identification Logic

Data Transfer Logic

Address and Range Logic

I/O CONTROLLER TIMING LOGIC

I/O CONTROLLER REQUEST LOGIC

I/O CONTROLLER INTERRUPT REQUEST LOGIC

I/O CONTROLLER REQUEST RESET LOGIC

DMA IOC REQUEST AND RESET LOGIC

I/O CONTROLLER SYSTEM BUS REQUEST AND LINK LOGIC SUMMARY

CPU LOGIC DETAILS

CONTROL STORE LOGIC DETAILS

ROS Address Generation Logic

Hardware Interrupt Logic

Main Memory Refresh Timeout Logic

Main Memory Parity Error Logic

Nonexistent Memory Detection Logic

Software Interrupt Logic

Boot PROM Logic

CPU LOGIC DETAILS

Data Transceiver Logic

Scratch Pad Memory Logic

Byte Swapping Logic

Microprocessor and Data Selection Logic

I, M1 and F Register Logic

Bus Command Logic

I/O Command Logic

Proceed and Busy Logic

Read/Write Byte Logic

Memory Go Logic

MAIN MEMORY DESCRIPTION

SYSTEM BUS INTERFACE

Data Word

Address Word

MAIN MEMORY ORGANIZATIONAL OVERVIEW

MODULE PHYSICAL/ORGANIZATIONAL CHARACTERISTICS

Module Addressing

MEMORY SAVE UNIT

MAIN MEMORY FUNCTIONAL OVERVIEW

Main Memory Timing

Main Memory Modules

Timing Generator

Negative 5 Volt Generator

Power Failure Logic

RAM Address Control and Distribution Logic

Segment Select Logic

Data In/Data Out Registers

Parity Generator and Check Logic

Read/Write Control Logic

Refresh Logic

Chip Select Logic

MAIN MEMORY SUMMARY

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT DESCRIPTION CONVENTIONS

In the system of the invention, electrical signals indicative of binarydigits (bits) are applied to and obtained from various logic gates orother circuit elements. For the sake of brevity in the discussion whichfollows, the bits themselves are sometimes referred to rather than thesignals manifesting the bits. In addition, for the sake of brevity, thesignal names are sometimes used to label the lines connecting thevarious logic gates and circuit elements. These signals are sometimesreferred to by a group of letters or numbers. For example, in FIG. 14,BCYCOT- at the upper right identifies a signal output by NAND gate 295.Sometimes a group of letters is followed by a plus sign or a minus sign.The plus sign means that when the signal represents a binary ONE (ortrue), it is a high level signal and the minus sign means that when thesignal represents a binary ZERO (or false), it is a low level signal. Insome cases the plus sign or minus sign may be followed by a couple ofletters or numbers to distinguish that signal name from similar signalmeans with identical name beginnings. For example, in FIG. 43, signalPROCED- is output by decoder 244-3, signal PROCED-2A is output byinverter 546 and signal PROCED-20 is output by NOR gate 550. When themeaning is clear, sometimes the plus sign or the minus sign or otherqualifying suffixes (letters or numbers) are omitted. For example, inFIG. 39 sheet 1, the suffix BA and BB are omitted from the signalRDDT29+ meaning that the signal would be RDDT29+BA if the I/O controlleris connected to system bus A and the signal would be RDDT29+BB if theI/O controller is connected to system bus B. In other cases, when themeaning is clear, the letter "X" is used in signal name to indicate oneof several signals. For example, in FIG. 39 sheet 1, the signal namesPINTRX- and PIOCTX- refer to signal PINTR1 and PIOCTA- if the I/Ocontroller is connected to system bus A and they refer to signalsPINTR2- and PIOCTB- if the I/O controller is connected to system bus B.In some cases, a series of signals is indicated by using a hyphen afterthe first signal name followed by the suffix of the last signal name.For example, in FIG. 42 sheet 2, the output of register 242 is 48signals named RDDT00+ through RDDT47+.

For the sake of simplicity, logic gates are referred to as AND, OR, NANDand NOR gates, the difference between an AND gate and a NAND gate beingthat the NAND gate has an inverter, designated by a little circle in thedrawing, on its output line. The presence of an inverter on the outputline is also used to distinguish between a NOR gate and an OR gate.Inverters on the input lines of gates do not affect the name given tothe logic gate. For example, in FIG. 42 sheet 1, gate 591 is referred toas an AND gate, gate 594 as a NAND gate (inverted output) and gates 584and 595 as NOR gates (both with inverted outputs with the inverters ongate 584 inputs ignored for reference purposes).

It is also assumed, for purposes of illustration, that logic requiringpositive inputs for a positive output is employed unless indicatedotherwise. That is, the logic circuits such as AND and OR circuits, forexample, are operated by high signal levels at the input to produce ahigh level signal at the output. Logic levels which are not high will betermed low.

SYSTEM BUS OVERVIEW

A block diagram of the system is shown in FIG. 1. The central processorunit (CPU) 200 controls the system bus. The system bus is composed oftwo buses named system bus A, 202 and system bus B, 204. System buses Aand B, 202 and 204, are used to interconnect the CPU 200, input/output(I/O) controllers 206, 208, 210 and 212, main memories or I/Ocontrollers 214, 216, 218 and 220 and the memory save unit 222.

For simplicity, FIG. 1 shows only four main memory or I/O controllersconnected to each system bus. In the preferred embodiment, up to eightI/O controllers can be connected to each system bus if the physicalpackaging (available printed circuit board slots) of the system permits.As will be seen later, the limitations of eight I/O controllers persystem bus is due to timing consideration and a change in the systemtiming could permit more or less I/O controllers per system bus.

The control panel 201 connects directly to the CPU 200. System bus B 204is similar to system bus A 202; however, system bus B containsadditional memory control signals which are not present on system bus A.Therefore, only I/O controllers may be installed on system bus A whereasmain memory or I/O controllers may be connected to system bus B. The I/Ocontrollers connected to the system buses are used to control theoperation of peripheral devices connected to the I/O controllers. Themain memory, which can connect only to system bus B, is used to storethe software programs which are processed by the CPU.

The control panel 201 connected directly to CPU 200 is used by thesystem operator to initiate, monitor and direct the operation of thesystem. The optional memory save units 222 provide the DC voltages tothe system's volatile semiconductor random access main memories. Duringnormal power up conditions, the memory save unit 222 operates from DCvoltages supplied by a local system power supply (not shown) generatingthe required memory voltages while keeping its rechargeable batteries atfull charge. During power outages, the memory save unit 222 provides anemergency capability for retaining the volatile semiconductor mainmemory contents by providing battery back-up for a period of time, forexample, 5 to 10 minutes, depending upon the amount of main memory beingpowered.

The system shown in FIG. 1 can be configured into a variety ofparticular configurations by choosing various combinations of mainmemory, I/O controllers and peripheral devices. One such example systemconfiguration is shown in FIG. 2. Now referring to FIG. 2, an examplesystem configuration having a CPU 200 connected to system bus A 202 andsystem bus B 204 is shown. FIG. 2 shows a central processor unit with64K words (1K=1024) of main memory, four diskette peripheral drives, aline printer four communication lines, a console device and a printerconnected as follows. Main memory 1, 214-1, containing 48K words andmain memory 2, 216-1 containing 16K words, are connected to system bus B204. Memory save unit 222 is also connected to system bus B 204. Systembus A 202 and system bus B 204 are connected to CPU 200, control panel201 is directly connected to CPU 200. Diskette peripheral devices 1 and2, 270-1 and 207-2, are connected to system bus A 202 via diskettecontroller 1, 206-1. Diskette peripheral devices 3 and 4, 221-1 and221-2, are connected to system bus B 204 via diskette controller 220-1.Communication lines 1 and 2 are connected to system bus A 202 viacommunications controller 210-1. Printer peripheral device 209 isconnected to system bus A via printer controller 208-1. The consoleperipheral device 213 is connected to system bus A 202 via consolecontroller 212-1. It should be noted that a like numbered element in onefigure refers to the same numbered element in another figure; forexample, control panel 201 in FIG. 2 refers to the same element as shownas control panel 201 in FIG. 1.

CENTRAL PROCESSOR DESCRIPTION

The Central Processor Unit (CPU) is a firmware directed processordesigned as the controlling element within the system. The CPU containsan internal bus with two ports: system bus A and system bus B whichinterconnect the CPU, I/O controllers and main memory (shown in FIGS. 1and 2). CPU firmware combined with system bus hardware provides controlfor I/O controller and main memory transfers. Data from any source isplaced on a system bus by CPU firmware command and a main memory accesscan only be initiated by the CPU whether the main memory access is beingperformed on behalf of the CPU or an I/O controller. Thus, the need forpriority resolution logic within each controller and main memory toresolve conflicting requests to use the system bus is eliminated.

The CPU I/O structure supports dialog between main memory and I/Ocontrollers on two types of I/O channels: Data Multiplex Control (DMC)channels and Direct Memory Access (DMA) channels.

For channel of either type DMC or DMA, the system maintains a next dataaddress (i.e., the address of the location in main memory where the nextunit of information transferred to or from a peripheral device, via anI/O controller is to be read from or written into main memory) and arange. The range is the count of the number of units of information tobe transferred between main memory via the CPU and the I/O controller(peripheral device). In the preferred embodiment the main memory isorganized into words containing two 8-bit bytes. The next data addressesare specified as byte addresses and the range is specified as the numberof bytes to be transferred.

For DMC channels, the CPU retains and manages the range and next dataaddress information within the CPU resident Scratch Pad Memory (SPM).For DMA channels, the range and next data address is maintained locallywithin the I/C controller. I/O controllers are designed exclusively forthe system and are either a DMC type or a DMA type. Channel assignmentis by the channel number in the software I/O instruction. The CPUsupports a predetermined number of DMC input/output channel pairs, whereeach input/output channel pair contains one input channel and one outputchannel. For example, there are 64 input/output channel pairs in thepreferred embodiment and they can only be used by DMC I/O controllers.However the DMC channel numbers may be assigned to a DMA or DMC I/Ocontroller, but not both in the same system.

The CPU supports operating software which includes visible registers,data formats, instruction sets, and trap and interrupt operations.Operator interface is via the control panel and the console peripheraldevice. The control panel permits operator access to initialize thesystem.

CPU MAJOR COMPONENTS

A major block diagram of the CPU functional areas is illustrated in FIG.3 and is described in the following paragraphs.

Control store 230 is the controller element of the CPU. It contains aread only memory which stores firmware microprograms. These firmwaremicroprograms contain the functionality necessary to control CPUoperations. Also contained within this area is all of the addressing anddecoding logic necessary to sequence through firmware microprograms andissue commands to the hardware in a step-by-step manner.

Microprocessor 232 is the primary processing element within the CPU. Itperforms all the arithmetic, compare, and logical product operations andis exclusively controlled by firmware microinstruction commands.

The I/O system bus area 234 contains all the drivers/receivers andcontrol circuits necessary for the CPU to communicate with the I/Ocontrollers. Two buses are available: system bus A and system bus B.Main memory can only be connected to system bus B 204. The system busarea 234 is controlled by hardware and firmware microinstructioncommands.

Scratch pad memory 236 is a read/write memory which provides temporarystorage for the CPU data. Maintained in this memory is the range andaddress information for DMC channels and various working registersnecessary for CPU operation. This scratch pad memory 236 is controlledby firmware microinstruction commands.

PROGRAMMING CONSIDERATIONS

This section describes the various CPU registers that are softwarevisible and defines the various data and address formats used by thecentral processor unit.

Program Visible Registers

There are 18 central processor registers visible to the programmer usingthe software instruction set. The format and significant bits of eachregister are shown in FIG. 4. These registers are as follows:

Software Visible Registers

Seven word operand registers (R1-R7). Three of these are also indexregisters (R1-R3). These registers are 16-bits each.

Eight address registers (B1-B7 and P). These registers are 16-bits each.

Mask (M1) register (eight bits controlling trace and overflow trapenable).

Indicator (I) register (eight bits; overflow, reserved for future use(RFU-not used), carry-out, bit test, I/O, greater than, less than andunlike signs).

Status (S) register (16 bits; privileged mode bit, processor ID (fourbits), priority level number (six bits)).

Word and Address Formats

This section defines the various word and address formats that are usedby the central processor unit and shown in FIG. 5.

All data word elements, such as a bit or a byte, are based on 16-bitmain memory words. The format of each word is defined from left to rightwith the first bit numbered 0 and the last bit number 15. Main memorydata elements may be accessed by instructions to the bit, byte, word ormultiword data item level. In all cases, the leftmost element is themost significant element of the word; e.g., bit 0 is the first bit, bit1 is the second bit, bits 0 through 7 are the first byte, bits 8 through15 are the second byte, etc. Multiword items require successive wordlocations; the lowest address is defined as the leftmost or mostsignificant part of the data item.

An address pointer is used to point to bit, byte, word or multiword dataitems. This address indicates the leftmost and most significant elementsof the data item. Within an array, data items are numbered left toright. CPU addresses, address registers and program counters contain16-bits and store word addresses. The rightmost bit (bit 15) of anyaddress field is the least significant bit of the word address and forall address fields are unsigned. The system can be configured foraddressing up to 128K bytes (1K=1024). Byte dependent addresses for DMCdata requests are stored in CPU scratch pad memory as a 17-bit address.The least significant bit (bit 16) is set when byte one is addressed.The address formats for a memory word and a memory byte are shown inFIG. 5.

Main Memory

Main memory can be configured from a minimum of 4 KW (kilowords) to amaximum of 64 KW. Memory consists of a read/write random access memory.The main memory is contained on memory boards installed on system bus B.Main memory size can be increased in 4 KW increments with a minimum of 4KW of main memory configured in the lowest 4 KW address space. Thememory size switch on the CPU must be set to correspond to the size ofthe main memory configured in the system so that the CPU can check formemory addresses that attempt to reference nonexistent main memory. Aswill be discussed in detail later, the CPU checks for references tononexistent main memory for all memory access whether the access isbeing done on behalf of the CPU (for software instructions or data), oron behalf of a DMA or DMC I/O controller (for data from or to aperipheral device).

CPU AND SYSTEM BUS INTERFACES

There are two external interfaces for the central processor with thesystem bus: system bus A interface and system bus B interface.

The system buses A and B are part of the system chassis and provide acommunications path between the CPU, main memory or the I/O controllers.These systems buses also distribute power to the controllers and mainmemory. The system buses A and B are nearly identical and containapproximately 50 wires, or signals, each, the difference being thatsystem bus B has a main memory interface in addition to the set ofsignals on system bus A. Refer to FIG. 6 for a list of signals.

SYSTEM BUS A

System bus A distributes power and provides a communications path fordata transfers and interrupts between the CPU and each I/O controller(IOC) inserted in the bus connectors on the system bus A side of thesystem chassis. The CPU controls system bus usage and allocates servicerequest cycles on a separate time basis. Each IOC on system bus A isonly allowed to request service (for data transfer or interruptpurposes) at a time that is unique to that I/O controller and is afunction of the position (relative to the CPU) of the I/O controller onsystem bus A. The operation of system buses A and B is describedhereinafter.

FIG. 7 shows the signals on the system bus A. System bus A contains twosignals (MEMVAL, BWAC60) which are unique to the CPU. These signals areonly present on the CPU chassis slot connector of system bus A. Allother signals on this bus are on identical pins of each bus connectorwith the exception of two positions. The BCYCOT-BA signal (system bus Acycle out time) pin of one bus connector is wired to the next busconnector's BCYCIN-BA signal (system bus A cycle in time) pin. In thisway a priority timing signal is passed from one IOC to the next IOC onthe bus. FIG. 6 indicates the functionality and source of each signal onsystem buses A and B.

SYSTEM BUS B

System bus B distributes power and provides a communications path fordata transfers and interrupts between the CPU and each main memory boardor I/O controller inserted in bus connectors on the system bus B side ofthe system. System bus B is similar to system bus A. However, itcontains three additional main memory control signals, PMEMGO, PMFRSH,PBSFMD, which are not present on the system bus A. Therefore, mainmemory boards can only be installed in chassis slots on the system bus Bside of the system. All other system bus B signals are similar to thesystem bus A but are driven by a separate set of drivers. No signalsunique to the CPU chassis slot connector are present on system bus B,each signal feeds all chassis slot connectors on system bus B. Theoperation and control of the system buses A and B are describedhereinafter. FIG. 6 gives the functionality and source of each systembus signal.

As in the case of system bus A, each I/O controller on system bus B isonly allowed to request service (for data transfer or interruptpurposes) at the time that is unique to the IOC and is a function of theposition (relative to the CPU) of the IOC on system bus B. Main memory,although located on system bus B, does not make service requests butmust still pass on the priority timing signal (BCYCOT-BB and BCYCIN-BB)for use by I/O controllers on system bus B. Although only one I/Ocontroller on a given system bus (A or B) can make a service request ata time, two service requests can be made simultaneously by I/Ocontrollers in the same relative (time slot) position, one on system busA and one on system bus B. For example, referring to FIG. 2, diskettecontroller 2, 220-1, on system bus B can make an interrupt request atthe same time that printer controller, 208-1, on system bus A makes aDMC data request. Priority between these simultaneous system busrequests, as well as other outstanding but unresponded to earlier systembus requests, are resolved by the CPU as described hereinafter.

It should be noted that printer controller 208-1 on system bus A is inthe second physical bus connector slot, relative to the CPU, and secondbus request time slot whereas diskette controller 2, 220-1, is in thefourth physical bus connector slot, relative to the CPU, but in thesecond bus request time slot on system bus B. The difference between thephysical bus connector slot and the bus request time slot on system busB is due to the fact that each main memory board occupies one physicalbus connector slot but does not occupy a bus request time slot becausemain memory never requests the system bus (i.e., main memory does notinitiate any data transfers on the system bus and therefore the prioritytiming signals BCYCOT-BB and BCYCIN-BB need not be delayed by the mainmemory board).

CPU HARDWARE DESCRIPTION

A block diagram of the central processor unit hardware is illustrated inFIG. 8.

Major CPU Functional Areas

The central processor hardware is divided into four major areas: controlstore, scratch pad memory, microprocessor and I/O system buses as shownin FIG. 3. The following paragraphs describe at a block diagram levelthe components that are in each area.

Control Store

Now referring to FIG. 9, the control store 230 is the primarycontrolling element in the system. It is comprised of a read onlystorage (ROS) memory containing firmware microprograms and associatedaddressing and decoding logic necessary to interpret thesemicroprograms. Firmware is the link between software-controlledprogramming and system hardware operations. Firmware contains all thefunctionality to control all control store, scratch pad memory,microprocessor and I/O system bus operations. These microprograms aremade up of firmware words (microinstructions) arranged in a logicalorder. Each firmware microinstruction word contains 48 bits of encodeddata, when when decoded, causes specific hardware operations. Every 500nanoseconds, a firmware word is cycled out of ROS memory and decoded todetermine the next firmware address, and to generate specific commandsto the microprocessor, scratch pad memory and I/O system buses. Byexecuting firmware microprograms in a sequential manner, hardwareoperations are performed in the proper order to accomplish the desiredcentral processor action. Many firmware microprograms may be executedfor one hardware activity (i.e., control panel operation, servicing aninterrupt) or the execution of one software instruction. An overview offirmware flow and a description of the firmware word is providedhereinafter.

An intermediate block diagram of control store is illustrated in FIG. 9to assist in the description of CPU hardware and each block is explainedin the following paragraphs. Referring now to FIG. 9, firmware is storedin a 1K location by 48-bit read only stores (ROS) memory 238. Eachlocation stores one firmware word which is permanently written in theROS memory at the factory and cannot be altered. When an address isapplied to this ROS memory the corresponding firmware word is read out.

Boot PROM 240 is a 1K by 8-bit ROS memory which is only used duringbootstrap operations. It contains software instructions which are loadedinto main memory during initialization. The output of boot PROM 240 iswired-ORed to bits 24 through 31 of the output of normal firmware ROSmemory 238. During a boostrap operation this boot PROM is enabled andbits 24 through 31 of the normal ROS are disabled.

Local register 242 is a 48-bit register that receives the addressedfirmware word from ROS memory. This data is strobed in at time PTIME0and denotes the beginning of a 500-nanosecond CPU cycle. Decoders 244 isa network of multiplexers and decoding logic which generates specifichardware commands according to the firmware word currently stored in thelocal register 242. The output of command decoders 244 is distributed tocontrol panel 201, control flops 258, miscellaneous flops 264 and toother CPU Logic (see FIG. 8). Decoders 244 control various indicatorlights on control panel 201.

Control flops 258 consists of four flip-flops (CF1 through CF4) whichcan be set and tested directly by the CPU firmware undermicroinstruction control. They are used to remember and test for certainconditions between firmware steps. For example, control flop 3 (CF3) isused during a DMC data transfer sequence by the CPU firmware to rememberon which system bus (A or B) the I/O controller requesting the datatransfer is located.

Miscellaneous flops 264 consists of other flip-flops which are directlysettable and/or testable by the CPU firmware. Included in the group offlip-flops are the firmware watchdog timer (WDT) flop, firmware realtime clock (RTC) flop and the power failure flop. Also included in thisgroup of miscellaneous flip-flops 264 is the PCLEAR flop and the PDMCIOflop which are used by the CPU firmware to remember the state of systembus signal PBYTEX set by the responding I/O controller during proceedtime to inform the CPU of the I/O controller type (DMA or DMC) during aCPU command sequence (see FIG. 20). The CPU uses the IOC type stored inthe PDMCIO flop during an input address or input range CPU command todetermine whether the IOC's address and range will be found within theCPU's SPM program channel table, as is the case for a DMC IOC, orreceived from the system bus after being placed there by a DMA IOC.

Address register 246 is a 10-bit register which stores the currentfirmware word address. Its output is used to address ROS memory 238. Thenext firmware word address is clocked into this register at time PTIME2(primary time 2) of each CPU cycle.

Control store address generator 248 selects the address of the nextfirmware word to be read out of ROS memory 238. All addresses are branchaddresses and can be either decoded directly from the current firmwareword (microinstruction) or can be forced by hardware interrupts.

Four types of firmware addresses are decoded directly from the firmwareword and CPU test conditions (see FIG. 35D): (1) unconditional branch(UCB) to the firmware address in bits 38 through 47 of the currentfirmware word; (2) branch on test (BOT) condition (2 way test branch)selects one of 32 firmware testable conditions and causes a branchaddress to one of the two firmware locations depending on the true orfalse state of the selected conditions; (3) branch on major test (BMT)(multiple test branch) is used by the firmware to decode softwareinstruction operation codes and address syllables, stored constants andsoftware interrupts by performing a 16-way branch on the selected testcondition; and (4) to return to normal (RTN) (hardware interrupt returnbranch) causes a branch to the firmware address stored in the hardwareinterrupt return register and is used to return to the normal firmwareflow at the completion of a hardware interrupt firmware sequence.

Hardware interrupt network 250 forces a branch address in the controlstore address generator 248 whenever immediate action by the CPU isrequired. A hardware interrupt address can be generated on each CPUcycle and a separate address is generated for each hardware interruptcondition. The hardware interrupt condition and priorities are shown inFIG. 9. Firmware can inhibit the detection of any or all hardwareinterrupt conditions. If a hardware interrupt occurs, the normal nextfirmware address is stored in the hardware interrupt return register252.

Hardware interrupt return register 252 is 10-bits wide and stores thenext normal firmware word address when a hardware interrupt occurs. Thisaddress is then used to reenter normal firmware flow at the completionof the hardware interrupt sequence.

Branch on test network 254 uses the current firmware word to select oneof 32 firmware testable conditions and inform the control store addressgenerator 248 of the true or false state of the selected condition.

Major branch network 256 generates the next firmware address when a BMT(16-way) branch is performed. The current firmware word (bits 40, 41, 42and 43) indicates if either an op-code, address syllable, constant orsoftware interrupt condition is used to form the next firmware address.

Software interrupt network 257 detects conditions that can interruptsoftware processing and generates a unique firmware address for eachcondition on a priority basis. Firmware uses these addresses to branchto the correct firmware routine to service the interrupt condition. Itshould be noted that BMT branch for testing software interruptconditions is only performed at the beginning of a software instructionfetch from main memory sequence. The following listing contains thesoftware interrupt conditions in the highest to lowest priority order:

(1) register overflow trap when an overflow occurs in a CPU register(R1-R7) during data manipulation and that register has its correspondingbit set in the trap enable mask register (M1);

(2) power failure when the power supply detects that a power loss willoccur in a minimum of two milliseconds;

(3) I/O interrupt system bus A when an I/O controller attached to thesystem bus A has requested an interrupt cycle;

(4) I/O interrupt system bus B when an I/O controller attached to thesystem bus B has requested an interrupt cycle; and

(5) timer interrupt when a fixed timed interrupt derived from the acline signal used to update a real time clock.

Returning now to FIG. 8, Scratch pad memory (SPM) 236 is a 256 locationby 17-bit random access memory used to store CPU status, I/O range andbuffer addresses for DMC channels. It also provides temporary storagefor data, addresses, constants and I/O byte transfers. Maintained in SPMare: 15 work locations, CPU status regster and program channel table(PCT).

Not all locations of the SPM are used. Refer to FIG. 10 for the scratchpad memory layout.

The 15 work locations (locations 00 through 06 and 08 through 0F,hexadecimal) are used for temporary storage. Some of the uses of thesework locations are temporary storage of I/O data before transfer tomemory, maintaining previous program address and intermediate storagefor byte swapping during DMC data transfers.

The CPU status register is location 07 (hexadecimal) of SPM. Thislocation is directly accessible by software. This location alwayscontains the current CPU status. See FIG. 4 for bit definitions.

The program channel table (PCT) occupies the upper (higher address) 128locations of SPM. It is used exclusively by the CPU to manage DMCchannel operations. The attributes of the program channel table are:

(1) it consists of 64 entries, where each entry can be used as either aninput or output channel because the input/output channels are halfduplex (i.e., either in the input mode or output mode at any instant intime), therefore only one entry is needed per input/output channel pair;and

(2) each entry consists of a 17-bit byte address and a 16-bit range andoccupies two consecutive SPM locations.

A PCT entry is loaded whenever an I/O load (IOLD) software instructionis directed to its associated DMC channel. Each time a DMC data transferoccurs the appropriate PCT entry is updated. Information can be readfrom a PCT entry via a software I/O instruction to a DMC channelspecifying in its function code field one of the following I/O commands;(1) input address; (2) input range; and (3) input module.

Scratch pad memory is directly controlled by the current firmware word(see firmware word description below). SPM write occurs at PTIME4 if bit0 of the firmware word is a binary ONE. Data input to SPM 236 is fromthe internal bus 260 via a byte swapping multiplexer 262 (see FIG. 8).In byte operations multiplexer 262 swaps the left and right byte of theSPM input data if the left byte of the word on the internal bus 260 isto be manipulated. For DMC channels, the firmware resets bit 16 of thePCT entry address pointer in the SPM to identify that the left byte of amemory word is being manipulated. Addressing of location within the SPMis also controlled by firmware. The SPM access address comes directlyfrom the firmware word when accessing work locations and the statusregister from decoders 244 in FIG. 9 via SPM address multiplexer 294 inFIG. 8. Otherwise, the DMC channel number is used when access to PCT isrequired in which case the SPM address comes from channel numberregister 296 via SPM address multiplexer 294 in FIG. 8. When performinga DMC data transfer operation, the CPU firmware uses the low order bitof the channel number (bit 9 in FIG. 24) to determine whether an inputor output operation is to be performed using the address and rangestored in the PCT entry for the associated input/output channel pair.

MICROPROCESSOR

Again referring to FIG. 8, all activity in the central processor centersaround the processing capabilities of the firmware controlledmicroprocessor 232. All arithmetic, compare and logical productoperations within the CPU are performed by the microprocessor 232 whichis composed of four cascaded 4-bit sliced microprocessors to form a16-bit microprocessor. In the preferred embodiment, microprocessor 232is composed of four type Am2901 microprocessors produced by AdvanceMicro Devices Inc., of Sunnyvale, California. Within the microprocessor232 is a 16-location by 16-bit register file 268, an eight function16-bit wide arithmetic logic unit (ALU) 266, shift logic andmiscellaneous logic necessary to support the microprocessorcapabilities.

Data input to the microprocessor 232 is the 16-bit output from the dataselector multiplexer 269. The multiplexer 269 can select data fromeither SPM 236 output, the internal bus 260, the contents of theindicator register 270 plus the M1 register 272 or, a constant from thecurrent firmware word from local register 242 (see FIG. 9). Input datato the microprocessor 232 can be stored in the register file 268 or workregisters within the microprocessor or it can be delivered via the ALU266 to the internal bus 260 as microprocessor output data. This isdetermined by the firmware input to the microprocessor.

Bits 8 through 19 of the current firmware word control themicroprocessor (see firmware word description below). These bits controldata inputs to the ALU, the function which the ALU is to perform, andthe destination of the results of the ALU. The microprocessor performs anew operation according to the firmware word at each CPU cycle (500nanoseconds in the preferred embodiment).

Maintained in the microprocessor 232 are 16 registers in register file268, of which 15 are visible to software (see FIG. 11). A four bitaddress supplied to the microprocessor is used to address the registerfile. This address can be selected from the function register (FR) 274or directly from the firmware word. The FR register 274 initially storesthe operation code and then contains various address syllables andconstants or can be incremented or decremented as determined by firmwareflow. For file addressing, the FR register 274 is divided into threesections, (FR0, FR2 and FR3) and any one of these sections can beselected by file address multiplexer 276 to be used to address themicroprocessor register file 268. Data input to the register file 268 isvia the ALU 266. Register file 268 output data can be delivered to theALU 266, an internal work register (Q), or the internal bus 260 asdetermined by the current firmware word.

Data output from the microprocessor 232 is wired ORed with data inputreceivers from the system buses A and B at the internal bus 260.Therefore, if either of the system buses A or B receivers are enabled,the output of the microprocessor is disabled. However, firmware testablesignals (ALU equals zero, SIGN, overflow, carry) are always outputtedfrom the microprocessor 232.

In addition to providing input data to microprocessor 232, the output ofdata selection multiplex 269 can be gated into the M1 register 272 andindicator register 270 under firmware control. Four bits of the outputof data selector 269 can also be gated into quality logic test (QLT)register 278 under firmware control. The output of QLT register 278controls the lighting of four LED indicators located on the CPU boardwhich are used to give the data processing system operator a visualindication of the success or failure of the quality logic testsperformed during system initialization.

Clock 281 provides the various timing signals (PTIME0 through PTIME4 andBCYCOT) used throughout the system (see FIGS. 14 and 15).

SYSTEM BUS CONTROL

FIG. 12 illustrates both system buses (A and B), data paths and controlsignal development. The principle elements are bus subcommand decoders244, CPU internal bus 260, separate receivers 284 and 288 and drivers282 and 286 for each I/O system interface data/address lines, and CPUcycle out time generator 280.

Firmware controls data flow over both system buses A and B and anytransfer that occurs through the 16-bit CPU internal bus 260. Duringeach CPU cycle the subcommand decoders 244 interprets the currentfirmware word and generates bus control subcommands which are valid fromtime PTIME1 through PTIME4 of the cycle. These decoded subcommandsenable specific data paths and cause data transfers as required byfirmware. The dialogs which can be performed over the system buses aredescribed in the systems bus operation section below. Basic system buscontrol is described in the following paragraphs.

Separate subcommands determine if either system bus receivers 284 or 288are enabled to place data on the internal bus 260. If both A and B busreceivers 284 and 288 are disabled, the output of the microprocessor 232is transferred to the internal bus 260. If data is to be sent to an I/Ocontroller, the appropriate CPU to system bus drivers 282 or 286 areenabled causing data at the internal bus 260 to be transferred to theenabled system bus data/address lines. If data is to be transferred froman IOC to the CPU, the appropriate enable I/O controller data driversignal (PENBSA- or PENBSB-) is decoded and sent via the I/O interface toall I/O controllers on the specific system bus. However, only the IOCthat requested the bus access places data on the system bus. A separatesubcommand is generated when main memory is to transfer data to the CPU.The appropriate CPU receiver path must also be enabled to transfer mainmemory data to the internal bus.

As mentioned before, all transfers are via the internal bus 260. Forexample, if firmware determines data is to be transferred from an IOC onsystem bus A to main memory on system bus B, it enables the data driversin the IOC on system bus A (via signal PENBSA-) and system bus Areceivers 288 causing data to be transferred from the controller to theinternal bus 260. If it is a Direct Memory Access (DMA) transfer,firmware simultaneously enables system bus B drivers 282 causing thedata at the internal bus 260 to be transferred to main memory. If it isa DMC transfer, internal bus data is first sent to SPM 236 (FIG. 8) forpossible byte swapping. During a later CPU cycle, the data is retrievedfrom SPM 236 via the microprocessor 232 and system bus B drivers 282 areenabled, causing data to be transferred from the internal bus 260 tomain memory on system bus B.

The CPU cycle out time signal (BCYCOT) is used to permit the requestingof a system bus for a data transfer or interrupt by an IOC. It ensuresthat only one I/O controller communicates with the CPU at one time. Thissignal is generated every four microseconds and is propagated fromcontroller to controller down each system bus. Each I/O controlleraccepts the pulse and delays it for 500 nanoseconds before passing it onto the next controller (see FIG. 13). The time in which an I/Ocontroller delays the signal is called cycle in time for that I/Ocontroller. As discussed hereinbefore, because main memory never makes adata transfer or interrupt request, main memory does not delay the cycleout time signal on the system bus. Instead main memory passes signalBCYCOT to the next main memory or IOC on system bus B without delay.During cycle in time is the only interval in which an I/O controller canrequest system bus access. If CPU firmware grants access, a link betweenthe CPU and the I/O controller is formed, preventing any other I/Ocontroller from access to the system bus.

During this period in which the CPU and IOC are linked, other I/Ocontrollers on the same or alternate system bus can make system busrequest during their cycle in time but the CPU will not grant access tothe system bus. This CPU-IOC link is done under firmware control byinhibiting software and hardware interrupts until the link is released.The CPU-IOC link is established and maintained by each firmwaremicroinstruction word of the microprograms used to process the datatransfer or interrupt request having a bit reset to inhibit hardware(and also software) interrupts. The first microinstruction with thehardware interrupt bit reset establishes the CPU-IOC link, and afterestablishment, the first microinstruction word with the bit set releasesthe link.

CONTROL PANEL

The control panel (201 in FIG. 9) connects directly to the CPU andallows the operator to manually initialize, bootload and start thesystem. The control panel includes a pushbutton (momentary) switch usedto start the initialize (bootload) sequence. Depressing the initializepushbutton switch resets the ROS address register (246 in FIG. 9 viacontrol store address generator 248) causing a branch to the initializefirmware routine. It also momentarily grounds signal PCLEAR- on bothsystem buses causing the I/O controllers to initiate quality logic tests(QLTs).

BASIC SYSTEM TIMING

Now referring to FIG. 14, the basic system timing is developed from a10-megahertz oscillator 290, the output of which, signal PCLOCK-, isconnected to the clock (C) input 5-bit shift register 291. Shiftregister 291 is the type SN7496 manufactured by Texas Instruments Inc.of Dallas, Texas and described in their publication entitled "The TTLData Book for Design Engineers", Second Edition and incorporated byreference herein. Shift register 291 has a binary ZERO at the presetenable (PE) input, a binary ONE at the clear (R) input and a binary ZEROat the preset inputs (S1 through S5). The output of AND gate 293 (signalPTIMIN+) is connected to the serial (D) input of shift register 291. Theoutputs of shift register 291, signals PTIME0+ through PTIME4+, are usedto produce a basic 500 nanosecond CPU cycle which is divided into 5equal 100 nanosecond time periods and is shown in FIG. 15. These timesare used throughout the system to strobe and gate specific events,specifically:

PTIME0 denotes the beginning of a CPU cycle. The addressed firmware wordis gated into the control store local register 242 and the decoders 244are enabled (see FIG. 9).

The beginning of PTIME1 enables all system bus control signals whichremain enabled through the end of PTIME4. The signal PTIME0+, which wheninverted is a binary ONE from PTIME1 through PTIME4, enables thespecific data paths within the system buses and internal buses viasubcommand decoders 244 (see FIG. 12).

PTIME2 is used to gate the next firmware address which is valid at thistime into the control store address register 246 (see FIG. 9).

PTIME3 is sent to all I/O controllers on the system buses andsynchronizes the CPU and I/O controllers. Bus data is valid at thistime.

PTIME4 is primarily used by the CPU microprocessor. Any writing orstoring of information within the microprocessor 232 and scratch padmemory 236 occurs at this time (see FIG. 8).

Returning now to FIG. 14, the operation of the basic system timing logicwill be briefly explained. Assuming initially that the outputs of shiftregister 291, signals PTIME0+ through PTIME4+, are a binary ZERO andthat the clock stall signal PFREEZ+ is a binary ZERO indicating that theclock is not to be stalled, the output of AND gate 293 (signal PTIMIN+)will be a binary ONE. With a binary ONE at the serial (D) input of shiftregister 291, the occurrence of the transition from a binary ZERO to thebinary ONE state of the clocking signal PCLOCK- from oscillator 290 willcause the output signal PTIME1+ to become a binary ONE which will inturn cause the output of AND gate 293 (signal PTIMIN+) to become abinary ZERO as shown in FIG. 15. With each succeeding clock pulse fromoscillator 290, one of the outputs of shift register 291 becomes abinary ONE and the other four outputs become (or remain) a binary ZEROas shown in FIG. 15. Each of the outputs of shift register 291 is fed toan inverter to provide the inverse of the timing signals (i.e., signalsPTIME0- through PTIME4-). For simplicity, only inverter 297 for signalPTIME0+ is shown in FIG. 14. Signal PTIME0+ is also used as the clocking(C) input to synchronous up/down binary counter 292. Counter 292 is ofthe type SN74LS169A manufactured by Texas Instruments Inc., of Dallas,Texas, and described in their heretofore mentioned publication. Counter292 in conjunction with NAND gate 295 is used to produce the CPU cycleout time signal BCYCOT- which is fed down both system buses (A and B)for use by I/O controllers on the system buses to insure that only oneI/O controller per system bus makes a request for that system bus at agiven time. By counting down eight PTIME0+ signal transitions from thebinary ZERO to the binary ONE state, counter 292 in conjunction withNAND gate 295 causes signal BCYCOT- to be a binary ZERO for one CPUcycle time (500 nanoseconds) followed by a binary ONE for seven CPUcycle times. Specifically: the load (L) input of counter 292 is set to abinary ONE so that data inputs D1 through D8 are ignored (i.e., not usedto preload the counter), both count enable inputs (P and T) are set to abinary ZERO enabling counting, and the up/down (U/D) input is set to abinary ZERO setting the counter to the count down mode. Thus, upon theoccurrence of the first transition of the clocking signal PTIME0+ fromthe binary ZERO to the binary ONE state the four outputs of counter 292(signals BCNTL1+ through BCNTL8+) become a binary ONE (counting downfrom zero to a binary fifteen) and the output of NAND gate 295 (signalBCYCOT-) becomes a binary ZERO. Upon the second occurrence of thetransition of the signal PTIME0+ from the binary ZERO to the binary ONEstate, the signal BCNTL1+ will become a binary ZERO and the output ofNAND gate 295 will become a binary ONE. Signal BCYCOT- will stay abinary ONE until the 9th occurrence of signal PTIME0+ transitioning fromthe binary ZERO to the binary ONE state at which time the signalsBCNTL1+, BCNTL2+ and signals BCNTL4+ will once again all be a binary ONEresulting in the output of NAND gate 295 becoming a binary ZERO. Therelationship between the CPU primary time signals PTIME0 through PTIME4and the CPU cycle out time signal BCYCOT- is shown in FIG. 13.

Now referring to FIG. 13, it can be seen that the CPU cycle out timesignal BCYCOT- (first controller cycle in) transitions from the binaryONE to the binary ZERO state at the leading edge of PTIME0 of the secondCPU cycle and transitions from the binary ZERO to the binary ONE stateat the leading edge of PTIME0 of the third CPU cycle. This is contrastedwith the second and subsequent controller's cycle in (BCYCOT-) signalswhich transition from the binary ONE to the binary ZERO state upon theoccurrence of the trailing edge of the PTIME3- signal and transitionsfrom the binary ZERO to the binary ONE state upon the next occurrence ofthe trailing edge of the PTIME3- signal. This difference results fromderiving the CPU cycle out signal by counting every eighth PTIME0, asexplained hereinbefore, whereas each controller's cycle out signal isderived by receiving the trailing edge of PTIME3 signal while the cycleout signal from the previous (neighbor toward the CPU) I/O controller isa binary ZERO, as will be explained hereinafter with respect to FIG. 40.The necessary condition met by the system as embodied to functionproperly is that only one IOC on a system bus sees the trailing edge ofPTIME3 while the cycle out signal from the neighboring IOC (or CPU) is abinary ZERO. It should be noted that at each point in time the BCYCOT-signal received by (for example) a second IOC on system bus A from thefirst IOC on system bus A is in the same state as the BCYCOT- signalreceived by a second IOC on system bus B from the first IOC on systembus B.

SYSTEM INITIALIZATION

The CPU will react to power-up or initialize signal as shown in FIG. 16.

Now referring to FIG. 16, entry is made to the CPU initializationsequence at block 300 if the occurrence of the power on signal from thepower supply is detected. Entry is also made from block 302 if the poweris already on and the initialize pushbutton on the CPU control panel ispushed.

In block 304 a master clear signal is sent to the I/O controller causingthe IOC to initialize their logic thereby clearing the system buses Aand B to invoke any self contained IOC quality logic tests (QLTs).Master clear also initializes the CPU logic and block 306 is entered.Block 306 initiates CPU firmware sequencing at control store ROS (238 inFIG. 9) location 0. In block 307 the CPU firmware tests to determine ifsequence entry was initiated by depression of the initialize pushbutton(i.e., via block 302) and if so a full initialization is to be performedand block 312 entered. If sequence entry was made upon the detection ofpower on via block 300, block 307 exits to block 308 and less than afull initialization may be made.

Block 308 tests if the content of main memory is valid (i.e., signalMEMVAL- is a binary ZERO indicating that the memory save unit has acharged battery so that main memory refresh voltage has been maintainedduring any power off period). If main memory is valid, only a limitedinitialization need be performed and block 310 is entered executing abranch to main memory location 0 and software execution is begun. Mainmemory location 0 contains the first word of the software start upprocedure. Block 310 then exits to block 324 with the softwareexecuting.

If main memory is not valid, or if sequence entry was made from theinitialize pushbutton, a full initialization is to be performed andblock 312 is entered. The CPU firmware QLTs (resident in ROS memory 238in FIG. 9) are executed in block 312. When the CPU firmware QLTs arecompleted, block 314 is entered and the software program in the bootPROM (240 in FIG. 9) is transferred to main memory (locations 100through 2FF hexadecimal) and is executed. In block 316, execution of thesoftware program loaded from the boot PROM results in the sizing of mainmemory, performance of a parity test on all available main memory andthe performance of an extended CPU QLT and I/O test. In block 318, theresults of the extended CPU software QLT tests are checked. If no errorwas detected by the extended CPU QLTs, block 320 is entered and thesoftware boot program loads the first record off the boot load deviceinto main memory location 100 (hexadecimal). In block 322, once thefirst record is loaded into main memory, a branch to main memorylocation 100 is executed and the CPU is running with the initializationsequence complete at block 324. If the extended CPU software QLT resultsin the detection of an error, block 326 is entered from block 318 andthe control panel check indicator on the control panel remainsilluminated and the CPU QLT indicators (LED light on the CPU board)indicate the error. Block 328 is then entered and the CPU halts.

If during software execution in block 324 an impending power failure isdetected by the power supply, block 330, software execution isinterrupted and block 332 is entered. In block 332, the CPU attempts toperform a power failure interrupt sequence including the context save ofthe software program executing at the time of the power failure. Beforethe context save, the CPU clears the system buses to get them free foruse by CPU to main memory data transfers. The context save results inthe volatile CPU registers, which will lose their information if poweris not maintained, being stored into main memory for preservation duringthe power off period. Approximately 2 milliseconds after the detectionof the impending power failure main memory will stop responding to CPUrequests causing the halting of software execution in block 334. CPUfirmware execution will also halt in block 334 when there is no longersufficient power. The later detection of power on by the power supply inblock 300 will cause the CPU to exit block 334 and perform a partial orcomplete initialization depending upon whether main memory has remainedvalid during the power off period.

SYSTEM BUS OPERATIONS

System bus operations transfers addresses, data, and control informationbetween the CPU and the I/O controller and main memory attached to thesystem (see FIG. 17 for data formats). All system bus operations arecontrolled by CPU timing and firmware sequences. This section describesthe sequence of events on the system buses (A and B) that occur when theCPU communicates with an I/O controller or main memory.

System bus operations can be initiated by either the CPU or an I/Ocontroller. The CPU initiates system bus dialog for the followingreasons: (1) all main memory accesses; (2) main memory refresh; and (3)function codes to I/O controllers. An I/O controller initiates systembus dialog for the following: (1) direct memory access (DMA) datatransfers; (2) data multiplex control (DMC) data transfers; and (3) I/Ocontroller interrupts. The main memory does not initiate any system busdialog.

All I/O controller initiated bus activity is on a separate time basis.Controller logic only permits an IOC to initially request a bus cycleduring that I/O controller's unique cycle in time (see FIG. 13) and ifno other IOC on that particular bus has already made the same type ofbus cycle request (e.g., an IOC on system bus B can not make a DMCrequest if another IOC on system bus B has already set the line PDMCR2to a binary ZERO but the fact that another DMC IOC on system bus A hasalready made a DMC request by setting line PDMCR1 to a binary ZERO willnot inhibit the DMC request on system bus B). Since I/O controllers canonly request the system bus during its unique cycle in time no prioritycircuits are required in the I/O controllers.

Since two I/O buses are available and I/O controllers can request thebus for different purposes, CPU firmware reacts in the following highestto lowest priority: (1) B bus requests a DMA transfer; (2) A busrequests a DMA transfer; (3) A bus requests a DMC transfer; (4) B busrequests a DMC transfer; (5) A bus requests an interrupt; and (6) B busrequests an interrupt.

By referring to FIG. 9 it can be seen that the four highest priority busrequests (DMA/DMC transfers) are treated as hardware interrupts andhandled by hardware interrupt network 250. The two lowest priority busrequests (interrupts) are treated as software interrupts and handled bysoftware interrupt network 258.

Each system bus operation is controlled by CPU firmware sequences.Specific firmware commands informing the I/O controllers that theactions required are issued over the system bus. These I/O controllercommands are issued on the RDDT lines (RDDT29, RDDT30, RDDT31 of FIG. 6)of the system bus and come directly from the current CPU firmware word.When firmware issues a command to an IOC, the encoded command is placedon the system bus RDDT lines and the command strobe line (PIOCTA onsystem bus A or PIOCTB on system bus B or both PIOCTA and PIOCTB, seeFIG. 7) is forced to a binary ZERO. This causes the IOCs on the systembuses to decode the command and the desired action is performed. FIG. 18lists all the CPU firmware I/O commands that can be issued to an I/Ocontroller.

As will be discussed hereinafter in greater detail, the I/O commandslisted in FIG. 18 are broadcast on the RDDT lines of both system bus Aand B independent of whether the I/O controller to which a command isdirected is on system bus A or B. In those cases in which the commandmust be directed to only one system bus, for example when answering aDMA request of an IOC on system bus B, only one command strobe line(PIOCTA on system bus A or PIOCTB on system bus B) is set to the binaryZERO state so that I/O controllers on only that system bus will see theI/O command. In other cases when the CPU is directing an I/O command toan IOC which may be on either system bus, for example, when initiating aCPU command (CPCMD) to an IOC, both command strobe lines (PIOCTA andPIOCTB) are set to a binary ZERO so that all I/O controllers will seethe I/O command.

MEMORY ACCESS

All memory accesses are generated by CPU firmware. It requires two CPUcycles (one microsecond total) to access memory. FIG. 19 shows thesequence of events and signals required to transfer data to/from memory.

During the first CPU (500 nanosecond) cycle, the write byte signals aregenerated (PWRTB1, PWRTB0). Signal PWRTB0 is a binary ONE if the leftbyte (zero) of the data is to be written into memory. Signal PWRTB1 is abinary ONE if the right byte (one) of the data is to be written intomemory. These signals can come from a DMA controller or CPU firmware. Ineither case they are valid from primary time one through time four ofthe initial CPU cycle. These signals inform memory to either read a wordor write the associated byte(s). At primary time three of the first CPUcycle the memory go pulse (PMEMGO) is generated. Along with the memorygo pulse, the word address (16 bits) is placed on the address/data lines(BUSB00) through BUSB15) of system bus B. This address can come fromeither a DMA I/O controller or the CPU. In all cases it passes throughthe internal bus from one system bus (A or B) to the other system bus (Bor A). The address (during the first cycle) and the data (during thesecond cycle) is placed on both system buses A and B via the internalbus. The placement of the address or data on both system buses occurseven when the address (or data) originates from a DMA I/O controller onsystem bus B or from the CPU as a matter of convenience to allow eithersystem bus to be monitored for addresses or data and is not otherwiserequired in these cases for proper system operation. The memory go pulsecauses the memory to accept the address and start its access sequence.If the access is due to DMA I/O controller request, the CPU examines theaddress against the maximum address allowed by the setting of the mainmemory configuration switch on the CPU. If the address is greater thanthe switch settings, it forces the memory error signals (PEMPAR andMEMPER) informing the I/O controller of the detection of a nonexistentaddress. The I/O controller than sets the correct bit in its statusregister. CPU initiated memory requests are checked for nonexistentmemory addresses prior to memory go and a trap 15 results.

During the second CPU cycle, data transfer occurs. If the CPU or I/Ocontroller is to receive data, the CPU enables the memory board datadrivers (PBSFMD) and at primary time 3, memory places the data on systembus B and via the CPU internal bus on system bus A. If memory wasperforming a full word read and detects a parity error, it forces thememory error signal (MEMPER). The CPU passes the error to system bus Awith a parity check error signal (PMMPAR). If the access was due to anI/O controller request, the controller sets an error bit in its status.Parity error detected on CPU requested main memory accesses cause a trap17. Any main memory error signals are reset at the next memory go of thenext main memory operation. If data is to be written into memory the CPUor I/O controller places the data on the system bus during the secondcycle and memory writes the data according to the write byte signalsinto the addressed location at primary time three.

MEMORY REFRESH

A main memory refresh cycle occurs if the CPU issues a memory go(PMEMGO) along with the memory refresh signal (PMFRSH) on system bus B.No other system bus dialog is required. The CPU issues a main memoryrefresh signal at least every 15 microseconds. If CPU firmwaredetermines main memory is not being used, it can issue the memoryrefresh signal at anytime, thus preventing the interruption of the CPUprocessing to issue a memory refresh.

FUNCTION CODE TO I/O CONTROLLER

The CPU transfer function codes to an I/O controller during theexecution of IO, IOH, IOLD software instructions, resulting in a 16-bitword being transferred to or received from the I/O controller. FIG. 20shows the sequence of events and signals required to perform this systembus operation.

The sequence is initiated by the CPU issuing a CPU command (CPCMD) onthe RDDT lines and placing the channel number and function code on theaddress/data lines (BUSX00) through BUSX15) of both system buses A andB. The CPU will wait a maximum of 1.2 milliseconds for a response fromthe I/O controller identified by the channel number. During this timethe CPU effectively stalls, no software interrupt can be honored butdata transfer requests are serviced. The CPU stall results from firmwarelooping within the CPU microprogram processing the software instruction(IO, IOH, IOLD) that caused the CPU command to be issued to the I/Ocontroller. This looping within the I/O software instruction prohibitsthe processing of other software instructions or responding to softwareinterrupts. During this looping, the CPU firmware is waiting for theproceed or busy signal (PROCED or PBUSY) from the I/O controller tooccur before the firmware counts down a time out counter stored in a SPMwork location. The following responses are possible.

No response is received if the CPU has attempted to access a nonexistentor defective resource. A CPU firmware timer detects this condition andtrap 15 results.

The addressed I/O controller is busy and it cannot presently accept acommand. In this case the I/O controller forces the busy line (PBUSY) toa binary ZERO causing the instruction to terminate.

The retry (wait) response is received if the addressed I/O controllercannot accept the new command because of a temporary condition withinthe I/O controller which is not related to the addressed channel number.The controller forces both the proceed (PROCED) and busy (PBUSY) linesto a binary ZERO causing the CPU to re-extract the current instructionand reinitiate the dialog.

The normal response is for the I/O controller to force the proceed(PROCED) line to a binary ZERO, signalling that the I/O controller isnot busy and that the CPU should complete the sequence. If the addressedI/O controller is a DMA type, it also forces signal PBYTEX low (a binaryZERO) to inform the CPU of the type of I/O controller responding.

On detecting a response from the I/O controller, the CPU will issue ananswer-command (ASCMD) command on the RDDT lines causing the I/Ocontroller to reset the busy/proceed lines. When the answer command isissued, the CPU and I/O controller are linked and the CPU firmware isdevoted to the CPU-I/O controller transfer.

During link time the CPU examines the range value if it is a DMAcontroller. If the range value equals zero, the CPU informs the IOC ofthis condition by issuing an end-of-range (EOFRG) command on the RDDTlines. Some DMC I/O controllers require this information, others ignoreit.

Approximately six microseconds after the CPU issues the answer command(ASCMD), it issues an end-of-link (EOFLK) command on the RDDT lines. Thetime interval between when the CPU issues the ASCMD command the EOFLKcommand depends on the number of CPU firmware steps (microinstructionwords) which must be executed for the particular function code sent inthe CPCMD command. Because the CPU and IOC are linked during this time,with the CPU firmware and system buses dedicated to the IOC sequence,and hardware interrupts inhibited, as a design parameter this time islimited to approximately six microseconds to guarantee systemresponsiveness to hardware interrupts and system bus requests. At thebeginning of the end-of-link time, if the function code was an inputtype, the CPU enables the IOC data drivers (PENBSX) and the data word istransferred over the address/data lines to the CPU. If the function codewas an output type, the CPU places the data on the system busaddress/data lines and the IOC strobes the data off the bus at primarytime 3 of end-of-link time. If the CPU is transferring a 17-bit addressto a DMA controller, line PBYTEX reflects the low order address bit(byte offset) during end-of-link time. The CPU-IOC link is reset as aresult of end-of-link being detected.

DMC DATA TRANSFER REQUEST

A DMC I/O controller initiates the DMC data transfer request sequencewhen the I/O controller requires a byte of data to be transferredto/from an I/O buffer in main memory. This request can only occur aftera software IOLD instruction has been issued to the DMC IOC initiating aninput/output operation. FIGS. 21A through 21D show the system bus dialogfor the DMC data transfer sequence.

Now referring to FIG. 21A, when a data transfer is required, the DMC I/Ocontroller informs the CPU firmware by forcing the DMC data request line(PDMCRX) to a binary ZERO on the system bus on which the requesting I/Ocontroller is located. The IOC is only permitted to force this line to abinary ZERO if the following two conditions are met: (1) the line is notalready activated by some other IOC on that particular system bus and(2) at primary time 3 of cycle in time for this IOC. Cycle in time(BCYCIN signal) ensures that only one IOC at a time on a particularsystem bus can start a data transfer sequence. The DMC request line(PDMCRX) remains set until the CPU responds.

Activation of the DMC request line causes a hardware interrupt of theCPU to the DMC request CPU firmware microprogram. When the hardwareinterrupt occurs, which is a function of other higher priority hardwareinterrupts and whether or not the CPU firmware is inhibiting hardwareinterrupts, the CPU acknowledges the DMC request by issuing ananswer-DMC request (ASDMC) command on the RDDT lines (RDDT29 throughRDDT31). At this time the CPU and IOC become linked. For approximatelythe next six microseconds, depending upon the number of CPU firmwaresteps involved in the DMC transfer, the CPU is dedicated to this DMCdata transfer and no other traffic will be allowed on either system busA or B except that associated with the DMC data transfer.

At the next cycle after ANSDMC, the CPU enables the I/O controllerdrivers (via signal PENBSX), thus informing the IOC to place its channelnumber on the address/data lines (BUSX00 through BUSX15). The channelnumber is used by the CPU firmware to access the program channel tablein scratch pad memory and also determines the direction of transfer.

During the next six to seven CPU cycles, the CPU obtains the memoryaddress and range information for this channel from the program channeltable. The range is decremented and the memory address incremented andstored in the program channel table. If the range is depleted by thisrequest, the CPU issues an end-of-range (EOFRG) command on the RDDTlines, informing the IOC that this is the last transfer (FIGS. 21Athrough 21D). If data is to be read from memory (FIGS. 21B through 21D)the CPU accesses memory, performs any byte swapping necessary andpositions the data on the system bus address/data lines in byte positionone (i.e., bits 8-15).

The CPU then issues an end-of-link (EOFLK) command on the RDDT lines.This indicates to the I/O controller that data from main memory is onthe data/address lines if reading from memory. The I/O controller takesthis data at primary time 3 of EOFLK is a memory read is beingperformed. If writing in main memory, the CPU enables the I/O controllerdrivers via signal PENBSX, the I/O controller places the data in byteposition one (i.e., bits 8-15) and the byte is transferred to SPM forpossible byte swapping and then the CPU performs a memory access towrite the data in main memory. End-of-link (EOFLK) causes the linkbetween the CPU and I/O controller to be terminated and resetting of theI/O controller so that it will no longer respond to certain system buscommands until another link is established between the CPU and the I/Ocontroller. The completion of the CPU firmware microprogram for the DMCdata transfer results in the enabling of hardware interrupts (DMA andDMC data transfer requests and main memory refresh time out) each ofwhich is pending will result in system bus traffic. It should be notedthat since all system bus traffic is under the control of CPU firmware,the termination of the CPU-IOC link is not sufficient to establish othersystem bus traffic without the CPU firmware also allowing hardwareinterrupts. For example, in FIG. 21A, during CPU cycles 8 and 9 nosystem bus traffic can occur on either system bus because the CPU isstill occupied executing the firmware microprogram for DMC inputtransfer from the IOC to memory.

DMA DATA TRANSFER REQUEST

A DMC I/O controller initiates a DMA data transfer sequence when the I/Ocontroller requires either a byte or word of data to be transferredto/from the I/O buffer in main memory. This request can only occur aftera software IOLD instruction has been issued to the I/O controller. FIG.22 shows the system bus dialog for this sequence.

The DMA I/O controller informs the CPU it requires a DMA data request byforcing the DMA request line (PDMARX) on the system bus on which therequesting I/O controller is located to a binary ZERO. This line(PDMARX) remains activated until a response is received from the CPU.The I/O controller is only permitted to set this line (at primary timethree) if the following two conditions are met: (1) the line is notalready activated by some other I/O controller on that particular systembus; and (2) at primary time 3 (PTIME3) of cycle in time (BCYCIN-abinary ZERO) for this I/O controller. Activation of the DMA request linecauses a hardware interrupt of the CPU to the DMA request microprogram.When this occurs, the CPU acknowledges the request by issuing ananswer-DMA-request (ASDMA) command on the RDDT lines (RDDT29 throughRDDT31) and enables the I/O controller drivers by setting the (PENBX)line on the appropriate system bus. The CPU and I/O controller arelinked and all system bus activity is dedicated to this DMA transferonly.

When the I/O controller detects the answer-DMA-request (ASDMA) commandit immediately does the following: (1) resets the request line (PDMARX);(2) places the memory word address on the address/data lines (BUSX00through BUSX15); and (3) gates the write byte signals on the bus linesPWRTB1 and PWRTB0. Note, if the controller is on system bus A, the CPUenables the address and write byte signals to system bus B for mainmemory use.

At primary time three of the answer-DMA-request (ASDMA) command cycle,the CPU issues a memory go signal (PMEMGO) and the main memory strobesusing the memory go signal. If the CPU detects the address is greaterthan that permitted by the memory configuration switch, located on theCPU board, it informs the I/O controller by setting the memory errorline (PMMPAR on system bus A and MEMPER on system bus B), causing anerror bit to be set in the I/O controller's status register.

In the CPU cycle following the answer command (ASDMA), the CPU issues anend-of-link command (EOFLK) on the RDDT lines specifying that the datatransfer is to take place. If it is a write operation, the CPU enablesthe controller drivers PENSBX and the I/O controller places the dataword on the address/data lines. If the I/O controller is on system busA, the CPU enables the data transfer to system bus B and main memory. Ifthe operation is a memory read, memory drivers are enabled by the CPUand main memory places the data on the bus (the CPU enables the data tosystem bus A if required) and the I/O controller takes the data atprimary time 3 of the end-of-link (EOFLK) cycle. If main memory detecteda parity error, it informs the I/O controller by setting main memoryparity error (MEMPER) line. If required, this error is passed to systembus A by the CPU on line PMMPAR.

During the CPU cycle immediately following the end-of-link signal, theCPU I/O controller link is terminated and the memory error signals(MEMBER and PMMPAR) are reset.

I/O CONTROLLER INTERRUPT

An I/O controller initiates a system bus I/O interrupt sequence whensome data transfer is complete or some device status changes. FIG. 23shows the dialog performed over the system bus.

It should be noted that the I/O interrupt is a software interrupt andnot a hardware interrupt. That is, an I/O interrupt, if accepted by theCPU, interrupts the execution of the current software program by forcingthe CPU to save the current state of the software. The CPU theninitiates the execution of other software dedicated to servicing the I/Ointerrupt. Upon completion of the I/O interrupt software, the state ofthe interrupted software is restored and the CPU continues executing theoriginal interrupted software program.

Where an interrupt is required the I/O controller informs the CPUfirmware by forcing the interrupt request line (PINTRX) to a binaryZERO. This line remains set until the CPU responds. The IOC is onlypermitted to activate this line at primary time 3 of that I/Ocontroller's cycle in time and if that line is not already activated bysome other IOC on the same system bus (A or B).

The activation of an I/O interrupt request line (PINTRX) on eithersystem bus causes the CPU to branch to the I/O interrupt firmware whenthe CPU firmware begins processing the next software instruction.

Software interrupts can only occur between the execution of softwareinstructions (i.e., the presence of a software interrupt will not beacted upon by the CPU during the execution of a software instruction butonly at the beginning of the next software instruction). This isaccomplished by microprogramming the CPU firmware used to implement theCPU software instructions to only branch on pending software interruptsonly at the beginning of the CPU firmware microprogram which fetches anddecodes the next software instruction from main memory. If an I/Ointerrupt is pending at the beginning of the execution of a softwareinstruction, the CPU firmware will abort the execution of the nextsoftware instruction and branch to the CPU firmware microprogram whichhandles the I/O interrupt processing. During the processing of the I/Ointerrupt the sequence shown in FIG. 23 occurs on the system bus. If theI/O interrupt is accepted, (i.e., the priority level of the IOC ishigher than the priority level of the currently executing softwareprogram) by the CPU, the CPU firmware saves the current software stateand begins executing the software program associated with the I/Ointerrupt. If the I/O interrupt is rejected, the CPU firmware continuesthe execution of the software instruction without interruption.

Now, referring to FIG. 23, when the CPU branches to the I/O interruptfirmware, the CPU acknowledges the request by issuing ananswer-interrupt (ASINT) command on the RDDT lines. This causes the IOCto reset the interrupt request line. The CPU and I/O controller arelinked and all system bus, main memory and CPU activity is dedicated toservicing the system bus I/O interrupt request.

Immediately after the answer-interrupt (ASINT) command, the CPUactivates the enable controller driver line PENSBX and the IOC placesits channel and interrupt level on the address/data lines. If the IOC isa DMC type and the interrupt is due to a backspace, the IOC informs theCPU by setting the PBYTEX- line to a binary ZERO when transmitting thechannel number and the interrupt level to the CPU on the system busaddress/data lines (BUSX00 through BUSX15). In the case of a backspaceinterrupt, the level is ignored and the interrupt is always accepted. Abackspace interrupt causes the memory address and range count in the PCTfor the associated DMC channel to be altered by the CPU firmware toignore the previous character.

If not a backspace, when the CPU receives the interrupt level itdetermines if the level presented by the IOC is of higher priority thanthe current level running in the CPU. If the IOC interrupt is of highpriority, the CPU will set the proceed line (PROCED) in conjunction withissuing an end-of-link (EOFLK) command on the RDDT lines and the link isterminated. If the controller interrupt is of lower priority (or equal),the CPU sets the busy line (PBUSY) in conjunction with issuing an EOFLK.In this case, the link is terminated and the IOC stacks the I/Ointerrupt and must wait until the CPU issues a resume-interrupt (RESUM)command on the RDDT lines.

The CPU issues a resume-interrupt (RESUM) command whenever a levelchange occurs. The RESUM command is broadcast on both system buses A andB and monitored by each I/O controller on the system buses. When an IOCwith stacked interrupts decodes a RESUM command, it sets an indicatorwithin the I/O controller so that during that IOC's cycle in time(BCYCIN time) the IOC reissues the I/O interrupt request and theinterrupt sequence starts again. The reissued I/O interrupt request willthen either be accepted or rejected. If the interrupt request isrejected (PBUSY is ZERO), the IOC will once again stack the I/Ointerrupt and wait for a RESUM command from the CPU.

Whenever a RESUM command is issued, each IOC with stacked interrupts oneach system bus will make an I/O interrupt request during its cycle intime if the interrupt request line (PINTRX) is not already set byanother IOC on that particular system bus. Because the I/O interruptrequest line is reset by the IOC in response to the ASINT command fromthe CPU, whether or not the CPU accepts or rejects the interrupt,another IOC can make an I/O interrupt request (i.e., set PINTRX to abinary ZERO) while the CPU is still processing the first I/O interruptrequest. This second I/O interrupt request will not be acted upon by theCPU until the CPU firmware starts processing the next softwareinstruction. The rejection of an I/O interrupt request, which results inthe interrupt being stacked in the requesting IOC, does not block orotherwise interfere with other I/O controllers on that particular systembus making their I/O interrupt requests (stacked or otherwise) followinga RESUM command. This is because a stacked interrupt will not be retrieduntil the IOC receives a RESUM command after stacking the I/O interrupt.Thus, each IOC will make an interrupt request for its stacked interruptfollowing each RESUM command (unless a second RESUM command occursbefore each IOC has had an opportunity to make an interrupt request).

It should be noted that the acceptance of software interrupt does notblock other software interrupts but only raises the CPU priority level.Therefore, another software interrupt request can be accepted during theprocessing of a first software interrupt if the second interrupt is ofhigher priority level than the first interrupt. This acceptance ofhigher levels can result in nesting interrupts to as many prioritylevels as there are waiting to execute at any given instant limited onlyby the requirement that the interrupting level be of higher prioritythan the interrupted level and by the number of levels in the CPU (64 inthe preferred embodiment).

EXECUTION OF INPUT/OUTPUT INSTRUCTIONS

There are three types of software I/O instructions supported by the CPU:IO, IOLD and IOH.

Execution of these instructions causes the CPU to initiate a dialog withthe I/O controller assigned to the selected channel and report tosoftware via the CPU's I indicator whether or not the IOC accepted thecommand. The I indicator is bit 12 of the indicator register (see FIG.4). If I=0 then the IOC did not accept the command. If I=1 then the IOCaccepted the command. A trap 15 occurs when no response was detectedfrom the I/O controller. A CPU firmware timer of 1.2 milliseconds timesout if a response (PROCED or PBUSY signal) is not received from the I/Ocontroller. During the 1.2 millisecond time out period, softwareinterrupt can not occur (because it is after the beginning of theexecution of the I/O software instruction) but hardware interrupts canoccur because they are not inhibited by the CPU time out firmware (seeFIG. 20). Once the CPU receives the proceed (PROCED) signal from theIOC, the CPU firmware inhibits hardware interrupts and issues the answer(ASCMD) command. The CPU firmware maintains the inhibiting of hardwareinterrupts until after issuing the end-of-link (EOFLK) command.

Channel Numbers

Input/output data transfer channels exist for each unit (CPU, I/Ocontroller or main memory) attached to the system buses with theexception of main memory which is identified only by a memory address.Channel numbers identify the I/O channel associated with the processor,peripheral devices, and if required, I/O controllers attached to thesystem. The CPU is always numbered channel zero. The first eight I/Ochannel pairs are reserved for use by the CPU (i.e., channel numbers0000 through 0300 hex) so that of the 64 DMC I/O channel pairs in thepreferred embodiment, 56 are actually available for the use ofperipheral devices connected to DMC I/O controllers although space isreserved for 64 channel pairs in the SPM program channel table (see FIG.10).

Software uses the channel numbers to identify to which I/O controller itwishes to direct a software I/O instruction. (The channel number iscontained in the control word of the software I/O instruction). Thechannel number is also used by the CPU to identify the direction of thedata transfer during data transfer time (i.e., bit 9 of the channelnumber determines direction).

FIG. 24 shows the software I/O instruction control word format. FIG. 24also gives the valid DMC channel numbers and valid DMA channel numbers.For both DMA and DMC, the first 8 channel pairs (16 channel numbers) arereserved for CPU use and are therefore not valid I/O channel numbers.Any software I/O instruction specifying a channel number which does notcorrespond to the channel number switch setting of an I/O controllerinstalled in the system will result in a trap 15 being posted by the CPUupon expiration of the CPU firmware timer.

The I/O controllers use the channel number to identify its channel whenit requires service from the CPU. Two types of services exist: (1)Interrupt--used by all (i.e., DMA and DMC) channels to signify its levelnumber to the CPU (see FIG. 17); and (2) Data--used by a DMC channel tosend or receive a byte. Note that the channel number is used by the CPUto address the program channel table (PCT). The channel number sent tothe CPU will have the format shown in FIG. 24. Note that for DMC datatransfer requests, bits 10 through 15 of the control word are ignored asshown in FIG. 17.

I/O Function Codes

The I/O function codes are presented in FIG. 25. The function codesspecify the specific I/O function to be performed by an I/O controller.All odd function codes designate output transfers (write) while all evenfunction codes designate input transfer requests (read). The functioncode commands always input/output either 16 bits (if issued via an IOinstruction) or eight bits (if issued via an IOH instruction) from/tothe channel. Nevertheless, I/O controllers can elect to use only part orall of the data bits involved in a transfer.

Output Function Code Commands

The eight output function code commands are described below.

Initialize (FC=01). This command will load a 16-bit control word to thechannel. Individual bits will cause specific action as follows: Bit0=initialize; Bit 1=1 stop I/O; and Bits 2 through 15=reserved forfuture use. This command will be accepted regardless of the channel busycondition. Initialize (IOC oriented): causes the IOC to run its residentquality logic test (QLT) (if any); clears all channels of the IOC;clears the bus interface; blocks interrupts; and clears the busycondition. Stop I/O (channel oriented): clears busy condition; causesinterrupt if enabled; abruptly stops transfer from channel; and does notaffect other channels on the same IOC.

Output Interrupt Control Word (FC=03). This command sends a 16-bitinterrupt control word to the channel specifying the CPU channel number(zero) in bits 0 through 9 and interrupt level in bits 10 through 15.The channel will store the interrupt control word in its interruptcontrol register. On interrupting, the IOC will return the interruptcontrol word as shown in FIG. 17.

Output Task (FC=07). This command outputs a task word (or byte) to thechannel. The meanings of individual bits are device specific. Outputtask is intended for those functions which have to be output frequently(e.g., read a disk record, etc.) as compared with relatively staticinformation which is output via the configuration command (see below).

Output Address (FC=09). This command outputs a 17-bit quantity which isused by the channel as the starting byte address to/from where a datatransfer will be made. If the command is directed to a DMA channel, thenthe byte address is sent over the bus to the channel and stored in theIOC. If the command is directed to a DMC channel then the byte addressis stored in the appropriate PCT entry in the CPU. The output address FCis used in conjunction with a software IOLD instruction. Usage of thiscommand with any of the other input/output instructions will causeunspecified results.

Output Range (FC=0D). This command outputs range information to thechannel within the range of 0 through 2¹⁵ -1. If the command is directedto a DMA channel, then the range is sent over the bus to the channel andstored in the IOC. If the command is directed to a DMC channel, then therange is stored in the appropriate PCT entry in the CPU. The outputrange FC is used in conjunction with a software IOLD instruction. Use ofthis command with any of the other input/output instructions will causeunspecified results.

Output Configuration Word A (FC=11). This command outputs configurationinformation to the channel. The meanings of individual bits are device(or IOC) specific. Output configuration is intended for those functionswhich are output only infrequently (e.g., terminal speed, card readermode, etc.).

Output configuration Word B (FC=13). This command outputs additionalconfiguration information to the channel. The meanings of the bits aredevice (or IOC) specific. This command is used where more information isrequired than can be coded into configuration word A.

Input Function Code Commands

The 10 input function code commands are described below.

Input Interrupt Control (FC=02). This command causes a channel to placethe contents of its interrupt control register on the system bus datalines (BUSX00 through BUSX15). The format of data is as shown in FIG.17. Note the channel number is that of the CPU (i.e., zero) and not thatof the responding IOC.

Input Task (FC=06). This command causes the channel to place thecontents of its task register on the system bus data lines (BUSX00through BUSX15).

Input Address (FC=08). This command causes a DMA channel to place thecontents of its address register (low order 16 bits) on the system busdata lines (BUSX00 through BUSX15). This command when directed to a DMCchannel causes the CPU to extract the address (residual) informationfrom the appropriate PCT entry in the CPU.

Input Module (FC=0A). This command causes a DMA channel to place thehigh order bit of its address register (right justified) on the systembus. This command when directed to a DMC channel causes the CPU toextract the high order bit of the address information from theappropriate PCT entry in the CPU. The high order address bit (or modulenumber) is right justified on the system bus and is placed on data lineBUSX15.

Input Range (FC=0C). This command causes a DMA channel to place thecontents of its range register on the system bus data lines (BUSX00through BUSX15). This command when directed to a DMC channel causes theCPU to extract the residual range from the appropriate PCT entry in theCPU.

Input Configuration Word A (FC=10). This command causes the channel toplace the contents of its configuration word A on the system bus datalines (BUSX00 through BUSX15).

Input Configuration Word B (FC=12). This command causes the channel toplace the contents of its configuration word B on the system bus datalines (BUSX00 through BUSX15).

Input Status Word 1 (FC=18). This command causes the channel to placeits first status word on the system bus data lines (BUSX00 throughBUSX15).

Input Status Word 2 (FC=1A). This command causes the channel to placeits second status word on the system bus data lines (BUSX00 throughBUSX15). Status bit definitions are IOC specific.

Input Device identification (FC=26). This command causes the channel toplace its 16-bit device identification number on the system bus datalines (BUSX00 through BUSX15). Each peripheral device type is assigned aunique identification number thereby enabling a software program toidentify the particular peripheral device type that is attached to eachchannel in a system.

Software Input/Output Instructions

The CPU recognizes and executes via firmware three types of software I/Oinstructions: (1) data word and command I/O instructions (softwareprogramming mnemonic of IO); (2) data byte (half word) and command I/Oinstructions (software programming mnemonic of IOH); and (3) address andrange load I/O instructions (software programming mnemonic of IOLD).

IO Instruction

The IO software instruction is used to send or receive control wordsto/from I/O controllers. The function code defined by the IO instructiondetermines the direction of the information transfer. Typically, thesoftware will use an IO instruction to pass to an IOC the necessary taskword and configuration information required to perform a data transfer.Also, upon completion of the I/O operation, software will again use theIO instruction to retrieve status and residual range information.

The format of the IO software instruction (and also the IOH instruction)is shown in FIGS. 26A and 26B.

The IO instruction specifies two quantities, namely: (1) data wordidentified by DAS; and (2) control word identifying the external channel(or device) and function it is to perform.

The control word may be imbedded directly in the IO instruction as shownin the format of FIG. 26A or it may be elsewhere in the software programand pointed to by the IO instruction as shown in the format of FIG. 26B.Now, referring to FIGS. 26A and 26B where:

OP=Instruction operation code field.

    ______________________________________                                        OP =        00000 (binary) for an IO instruction                              =           00010 (binary) for an IOH instruction.                            ______________________________________                                    

DAS=Data address syllable. It specifies a location in main memory or CPUregister from/to which a word (for IO instruction) or byte (for IOHinstruction) is transferred to/from the I/O channel. The data addresssyllable (DAS) can have one of three formats:

Register address syllable in which a CPU register is the source ofdestination for the operation.

Immediate operand address syllable in which an operand of appropriatesize (word or byte) is imbedded directly in the instruction.

Memory address syllable in which an address of a location in main memorycontaining the operand is specified.

CH=Channel number or I/O device address.

F=Function code, which is IOC or device specific under the constraint:

If F is even, data will be transferred from the IOC to the CPU/memory(e.g., read status)

If F is odd, data will be transferred from the CPU/memory to the IOC(e.g., load control register).

CAS=Control word address syllable, pointing to control word containingCH and F. The format for CAS is the same as DAS.

Execution of the IO software instruction is controlled by CPU firmware.See FIG. 28 for a flowchart of the IO instruction. System bus operationsduring IO instruction execution were described above.

When executing an I/O instruction directed to a DMC channel, the CPUwill check the function code to determine if it is one of the following:

Input Module (one bit module number)

Input Address (16 bit byte address)

Input Range

If it is one of the above function codes, the CPU directly executes allor part of the instruction since it manages this information in scratchpad memory. If it is not one of the above function codes, the data willbe transferred to or received from the I/O controller.

When executing an I/O instruction directed to a DMA channel the CPUpasses the function code to the I/O controller and either receives orsends one word of information to/from the I/O controller.

Now referring to FIG. 28, the CPU firmware used to implement the IOsoftware instruction will be discussed in detail. Before describing FIG.28 it should be noted that there is not necessarily a one to onecorrespondence between the block shown in the flow chart and the numberof CPU firmware microinstructions used to implement each block. Forexample, multiple CPU firmware microinstructions are used to perform thefunction shown as block 904 and one CPU firmware microinstruction isused to perform the function shown as blocks 914 and 920.

The CPU firmware begins processing the IO software instruction at block901 where the firmware fetches the first word of the softwareinstruction from main memory. Following the reading of the first word ofthe software instruction from main memory, the CPU firmware does a testto see whether any software interrupts are pending (not shown in FIG.28, but see FIGS. 33 and 34) . If so software interrupt is pending,block 902 is entered and the program counter which points to the firstword of the software instruction is saved, a memory refresh operation isinitiated because the memory will not be accessed during the next CPUcycle, and a test of the operation code of the software instructionfetched from main memory is performed. If the operation code of thesoftware instruction fetched from main memory is an IO softwareinstruction, 903 is entered, and the CPU firmware routine associatedwith the execution of the IO instruction is begun. In block 904 the CPUfirmware determines the word address using the data address syllable(DAS) (see FIG. 26). If indexing is specified, then the index value willbe in terms of words, as opposed to bytes. The word address specified bythe DAS specifies a location in main memory or a CPU register from/towhich a word of data is to be transferred to/from the IOC. The specifiedlocation contains a word of data to be sent to the IOC or is where theword of data read from the IOC is to be stored. In block 905 the CPUfirmware determines the channel number and function code specified bythe IO software instruction using CAS if required (see FIG. 26B). Inblock 927 a test is made to determine if the function code is an outputfunction code and if so the data word specified by DAS is accessed andstored in a scratch pad memory work location. In block 906 the channelnumber and function code are sent to the IOC on the system busdata/address lines (BUSX00 through BUSX15) (see FIG. 17, I/O commandformat). In addition in block 906, the CPU sends the CPCMD command onthe system bus RDDT lines indicating to all IOCs that a channel numberand function code are present on the system bus (see FIG. 20).

Following the CPU sending the CPCMD command, the firmware enters I/Oinstruction proceed firmware, block 948, which loops until the addressed(by channel number) IOC accepts or rejects the CPCMD command or informsthe CPU to retry the I/O software instruction. I/O instruction proceedfirmware block 948 begins by initializing a response timeout counter inblock 907. This response timeout counter is contained in a scratch padmemory work location and is counted down as the firmware loops waitingfor the IOC to acknowledge CPCMD command from an input/output (IO, IOHor IOLD) software instruction. If the IOC does not acknowledge or rejectthe CPCMD command within 1.2 milliseconds, the response timeout counterwill be decremented to zero and the CPU firmware will cause a trap (seeFIG. 20). After initializing the response timeout counter, the CPUfirmware tests whether the IOC has set the system bus PROCED line low inblock 908. If the PROCED line is not low, block 909 is entered and atest is performed to see if the IOC has set the PBUSY line on the systembus low. If the system bus PBUSY line is not low, block 910 is enteredand the response timeout counter is decremented. In block 911 a test ismade to see whether the response timeout counter is equal to zero and ifyes the 1.2 millisecond timeout has expired indicating that the CPCMDcommand was directed to a nonexistent, or malfunctioning I/O controller(i.e., no IOC on either system bus A or B responds to the channelnumber). If no IOC on either system bus A or B responds, block 912 isentered and a zero is set in the input/output indicator (I) bit ofindicator (I) register. This input/output indicator bit in the indicatorregister may be tested by subsequent software instructions to determinewhether or not the previous I/O command was accepted. After setting theinput/output bit indicator, block 913 is entered and a trap number 15 isperformed indicating that an unavailable resource has been addressed andexecution of the I/O software instruction is terminated.

Returning now to block 908, if the IOC sets system bus PROCED line low,block 919 is entered and the PBUSY line is tested. In block 919, ifsystem bus line PBUSY is set low by the IOC, it indicates that the IOCis temporarily busy and that the CPU should retry the instruction (i.e.,a wait condition). If a wait condition exists, block 920 is entered andthe CPU firmware sends the IOC an ASCMD command on the system bus RDDTlines which establishes the CPU-IOC link. This is followed by the CPUsending a EOFLK command in block 921 to the IOC on the system bus RDDTlines which terminates the CPU-IOC link (see FIG. 20). After terminatingthe CPU-IOC link, block 922 is entered and the program counter savedearlier is backed up to point to the first word of the I/O softwareinstruction currently being executed. Block 923 is then entered and theCPU firmware goes to fetch the first word of the software instructionpointed to by the program counter. This will result in the softwarerefetching and reexecuting the same I/O software instruction whichreceived the wait response from the I/O controller.

Because the I/O software instruction is retried from the beginning, caremust be taken not only to restore the program counter but also any otherCPU register which could be changed during a previous execution attemptwhich could receive a wait response from a temporarily busy IOC. In thepreferred embodiment, this is accomplished by not permitting forms ofoperand addressing which result in automatic incrementation ordecrementation of registers used in address development. Thisrestriction eliminates the need to restore any register other than theprogram counter.

Returning now to block 909, the IOC has set the system bus PBUSY linelow, block 914 is entered and a test is made to see whether the PROCEDlines is also low. If both PBUSY and PROCED lines are low, block 920 isentered and the CPU establishes and then terminates the CPU-IOC link andthen reexecutes the I/O software instruction as described above. Inblock 914 if system bus PROCED line is not low, indicating that the IOCis busy, block 915 is entered. Again in block 915, the input/outputindicator (I) bit in the indicator (I) register is set to zero fortesting by subsequent software instructions to indicate that an IOC didnot accept the last I/O command. Block 916 is then entered and the CPUsends the ASCMD command to the IOC to establish the CPU-IOC link. Inblock 917 the CPU firmware sends the IOC an EOFLK command on the systembus RDDT lines thereby terminating the CPU-IOC link. Block 918 is thenentered and the CPU firmware goes to fetch the next software instructionfrom main memory.

Before leaving the discussion of the I/O instruction proceed firmware,948, it is important to note that during the time that the CPU firmwareis looping (i.e., waiting for an I/O controller to respond with aproceed, busy, wait or timeout) and that although hardware interruptsare inhibited between some of the firmware microinstructions within theloop of blocks 908 through 911, the hardware interrupts are permitted atthe end of each pass through the loop. Therefore the CPU firmware may beinterrupted and the system bus may be used to perform a DMA datatransfer, a DMC data transfer, or a memory refresh operation. As will beseen later with respect to FIG. 34, no software interrupt will beresponded to during this 1.2 millisecond timing operation becausesoftware interrupts are tested for by the CPU firmware only immediatelyfollowing the fetch of a software instruction from main memory (i.e.,only after block 901).

When the CPU firmware detects a proceed condition (i.e., the respondingIOC has set system bus PROCED line low and left PBUSY line high) block924 is entered. In block 924 the CPU firmware sets a binary ONE in theinput/output indicator (I) bit of the indicator (I) register and alsostores the state of the system bus PBYTEX line in the PDMCIO flip-flopfor later testing of the IOC type. If the system bus PBYTEX line is highit indicates that the responding IOC is a DMC IOC and if low itindicates that the responding IOC is a DMA IOC. CPU firmware then sendsthe responding IOC an ASCMD command in block 925 on the system bus RDDTlines establishing the CPU-IOC link. In block 926 the function code istested. If the function code is odd, indicating an output function code,block 928 is entered and the CPU firmware gets the word of data pointedto by the data address syllable (DAS) from the SPM work location. Thedata word is sent to the IOC on the system bus address/data lines(BUSX00 through BUSX15) and the CPU also sends the EOFLK command to theIOC on the RDDT lines. When the IOC sees the EOFLK command it takes thedata word from the address/data lines and terminates the CPU-IOC link.Block 928 completes the processing of the IO software instruction andthe CPU then exits to block 929 which goes to the CPU firmware to fetchthe next software instruction.

From block 926, if the function code is even, indicating an inputfunction code, block 930 is entered and a test is performed to determinethe IOC type. If the responding IOC is a DMC IOC, block 931 is enteredand a further test is made to determine whether the function code isprogram channel table (PCT) related. If the function code from the IOsoftware instruction is program channel table related (i.e., inputmodule, input address, or input range), block 932 is entered and thedata specified by the function code is extracted from the programchannel table stored in the scratch pad memory and an EOFLK command issent to the IOC terminating the CPU-IOC link. Again returning to block930, the IOC type is determined by testing the state of the PDMCIO flopwhich was loaded in block 924 above. If the IOC is a DMA IOC block 933is entered. In block 933 the IOC places the data on the address/datalines (see FIG. 17, data word format) of the system bus upon receivingthe EOFLK and enable (PENBSX) signal from the CPU (see FIG. 20). Inblock 934 the data, either extracted from the program channel table(PCT) or received from the IOC, is stored in the location specified bythe data address syllable (DAS) of the IO software instruction. The IOsoftware instruction is completed with the storing of the data word inthe DAS location and the CPU firmware then goes and fetches the nextsoftware instruction in block 935.

Before leaving the discussion of the IO software instruction it shouldbe noted that, from the software point of view, the results of executingthe IO software instruction are the same whether or not the I/Ocontroller is a DMA I/O controller or a DMC I/O controller. Thedifferences between the DMA and DMC I/O controllers are masked from thesoftware by the CPU firmware. The software programmer need not becognizant of whether the device for which he is writing an input/outputprogram is attached to a DMA or DMC I/O controller.

IOLD Instruction

The IOLD software instruction is used to prepare for the data transferto/from an I/O buffer within main memory. For some I/O controllers, theIOLD software instruction also initiates the data transfer. For DMC I/Ocontrollers the CPU always ensures that the I/O buffer is containedwithin the configured main memory. If not, then an unavailable resourcetrap (TV 15) results.

The format of the IOLD software instruction is shown in FIGS. 27A and27B.

The IOLD instruction specifies three quantities, namely: (1) I/O bufferstarting address identified by AAS; (2) control word identifying theexternal channel (or device) and function it is to perform; and (3) I/Obuffer range (size) identified by RAS.

The control word may be imbedded directly in the IOLD instruction asshown in the format of FIG. 27A or it may be elsewhere in the softwareprogram and pointed to by the IOLD instruction as shown in the format ofFIG. 27B. Now, refer to FIGS. 27A and 27B where:

OP=Instruction operation code field.

OP=00011 (binary) for an IOLD instruction

AAS=Address address syllable. It specifies a byte location in mainmemory from/to which one or more bytes is transferred to/from the I/Ochannel. The address address syllable (AAS) has two formats:

Immediate operand address syllable in which an I/O buffer of theappropriate size (one or two bytes) is embedded directly in theinstruction.

Memory address syllable in which the address of the starting byte of theI/O buffer in main memory is specified.

CH=Channel number or I/O device address.

F=Function code of 09 (hex) which specifies output address. During theexecution of the IOLD software instruction by the CPU firmware, afterthe address is output, the CPU firmware changes the function code to OD(hex) and outputs the range.

CAS=Control word address syllable, pointing to control word containingCH and F. The format for CAS is the same as DAS (see IO softwareinstruction).

RAS=Range address syllable. It specifies a location from which the rangeof the I/O buffer, in terms of number of bytes of data, is to betransferred to the I/O channel. The format for RAS is the same as DAS(see IO software instruction).

Execution of an IOLD instruction is controlled by CPU firmware. See FIG.29 for a flowchart of software IOLD instruction execution. System busoperation during IOLD execution and the I/O controller request for I/Obuffer transfer after execution were described above.

IOLD instruction execution by the CPU firmware varies as determined bythe IOC type it is directed to, specifically:

IOLD instructions directed to DMA I/O controllers will cause two systembus transfers: The first transfer is a 17-bit byte address whichspecifies the I/O buffer starting location in main memory; the secondtransfer is a 16-bit range value which specifies the I/O buffer size interms of the number of bytes to be transferred during the I/O operation.This appears to the channel as two separate bus transfers and they havefunction codes of 09 (hex) for the address transfer and OD (hex) for therange transfer. The programmer need only specify the first function codein the IOLD instruction control word and the CPU firmware will generatethe second function code.

Address Transfer--The first transfer is a 17-bit quantity which is usedby the channel as the starting byte address for data transfers to/fromthe I/O buffer.

Range Transfer--The second transfer is a 16-bit range value whichrepresents the number of bytes to be transferred during the DMAoperation. The range is a positive integer data word where the rangevalue can be 0000 through 7FFF (hex). When this range transfer occurs,it also conditions some I/O controllers to initiate a data transferrequest to the I/O buffer. The data transfer is initiated in other I/Ocontrollers by use of IO software instructions with IOC specificfunction codes.

IOLD instructions directed to DMC IOCs will cause the CPU to store the17-bit byte address and 16-bit range value in the appropriate programchannel table entry. (See FIG. 30.) Upon completion of the I/Ooperation, the residual address and range will be available in the PCT.The CPU will also generate the two transfers which send the bufferaddress and range values to the I/O controller over the system bus. Ifthe programmer coded a range value of zero some I/O controller specificaction may be required. To inform the DMC IOC of this condition, the CPUduring execution of an IOLD directed to a DMC channel will send an endof range signal to the I/O controller. Some DMC I/O controllers ignorethis condition.

Now referring to FIG. 29, the CPU firmware used to implement the IOLDsoftware instruction will be discussed in detail. Again as describedwith respect to FIG. 28, in FIG. 29, it should be noted that there isnot necessarily a one to one correspondence between the blocks shown inthe flow chart and the number of CPU firmware microinstructions used toimplement each block.

The CPU firmware begins processing the IOLD software instruction atblock 901 where the firmware fetches the first word of the softwareinstruction from main memory. Following the reading of the first word ofthe software instruction from main memory, the software does a test tosee whether any software interrupts are pending (not shown in FIG. 29but see FIGS. 33 and 34). If no software interrupt is pending, block 902is entered and the program counter which points to the first word of thesoftware instruction is saved, a memory refresh operation is initiatedbecause the memory will not be accessed during the next CPU cycle, and atest of the operation code of the software instruction fetched from mainmemory is performed. If the operation code of the software instructionfetched from main memory is an IOLD software instruction, block 942 isentered and the CPU firmware routine associated with the IOLD softwareinstruction is executed. It should be noted that blocks 901 and 902 ofFIG. 29 are the same firmware instructions as shown in blocks 901 and902 of FIG. 28.

In block 943 the CPU firmware determines the byte address using theaddress address syllable (AAS) (See FIG. 27). If indexing is specifiedthen the index value will be in terms of bytes, as opposed to words. Thebyte address specified by the AAS specifies the beginning byte locationin the main memory of the I/O buffer from/to which the data transferto/from the IOC is to take place (see FIG. 30). In block 944, the CPUfirmware determines the channel number and function code specified bythe IOLD software instruction using CAS if required (see FIG. 27B). Inblock 945 a test is performed on the function code to determine if it is09 (hex) and if not, a trap 16 is performed by block 949 indicating aprogram error and execution of the IOLD software instruction isterminated. If block 945 determines that the function code is the outputaddress function code (09 hex) block 946 is entered.

In block 946 the CPU firmware uses the range address syllable (RAS) todetermine the range (in number of bytes) of the I/O transfer and storethe range in a SPM work location. In block 947, the channel number andfunction code are sent to the IOC on the system bus address/data lines(BUSX00 through BUSX15) (see FIG. 17, I/O command format). In addition,in block 947, the CPU sends the CPCMD command on the system bus RDDTline indicating to all IOCs that a channel number and function code arepresent on the system bus (see FIG. 20). Following the CPU sending theCPCMD command, the firmware enters I/O instruction proceed firmware,block 948 in FIG. 28, which loops until the addressed (by channelnumber) IOC accepts or rejects the CPCMD command or informs the CPU totry the I/O software instruction again (i.e., wait). If the addressedIOC acknowledges the CPCMD command, block 948 exits to block 951.

When the CPU firmware detects a proceed condition (i.e., a respondingIOC has set system bus PROCED line low and left PBUSY line high) block951 is entered. In block 951 the CPU firmware sets a binary ONE in theinput/output indicator (I) register and also stores the state of thesystem bus PBYTEX line in the PDMCIO flip-flop. If the system bus PBYTEXline is high, it indicates that the responding IOC is a DMC IOC and iflow, it indicates the responding IOC is a DMA IOC (see FIG. 20). CPUfirmware then sends the responding IOC an ASCMD command on the systembus RDDT lines establishing the CPU-IOC link. In block 953 the functioncode is tested. If the function code is 09 (hex) indicating an outputaddress function code, block 954 is entered. In block 954 the functioncode is augmented by four, changing the output address function code of09 (hex) to an output range function code of 0D (hex). By augmenting thefunction code from 09 to 0D, when the firmware is executed again for theoutput range function of the IOLD software instruction after entering atblock 947, block 953 will take the output range path and go to block964.

After augmenting the function code, block 955 is entered and the PDMCIOflip-flop is tested to determine the type of IOC that responded to theCPCMD command. If the responding IOC is a DMC IOC, block 956 is enteredand the IO buffer starting byte address determined by AAS is stored intothe program channel table entry associated with the specified channelnumber in the scratch pad memory (see FIG. 30). Also, in block 956 thebeginning byte address of the IO buffer is placed on the system busaddress/data lines (BUSX00 through BUSX15) with system bus line PBYTEXindicating whether it is byte zero or byte one. In addition, block 956sends the EOFLK command to the IOC indicating that data for the IOC ison the system bus. Although the beginning byte address of the buffer isbroadcast on the system bus for DMC as well as DMA IOCs, DMC IOCs ignorethe beginning byte address. The sending of the EOFLK command in block956 to the IOC terminates the CPU-IOC link established by the ASCMDcommand sent in block 952. In block 957 the CPU firmware determines theupper bound of the I/O buffer by taking the I/O buffer beginning byteaddress and adding the range (in bytes) of the I/O buffer.

In block 958, the CPU firmware determines whether the last byte of theI/O buffer exists in the main memory physically present within thesystem by doing a memory refresh operation addressing the last word(containing the last byte) of the I/O buffer. If the CPU logic detectsan attempt to address a nonexistent memory location, a hardwareinterrupt will be caused and block 970 will be entered and branch totrap 15 (block 961) which will terminate the execution of the IOLDsoftware instruction. If the last byte of the I/O buffer is contained ina main memory location physically present within the system, block 959is entered and a further check is made to see whether the I/O bufferwrapped around the high end of the 64K word memory and into the lowaddress memory locations. If wrap around occurred, block 961 is enteredand a trap 15 (unavailable resource) is executed and the execution ofthe IOLD software instruction is terminated. If the I/O buffer did notwrap around, block 959 exits to block 960 which reenters the IOLDfirmware and outputs the range. Block 960 goes to block 950 which entersblock 947 and begins the processing at this time of the output range (ODhex) function code.

Returning now to block 955, if the IOC type is a DMA IOC block 962 isentered. In block 962 the beginning byte address of the I/O buffer issent to the DMA IOC on the system bus address/data lines (BUSX00 throughBUSX15) (see FIG. 17, memory address format) with system bus line PBYTEXindicating byte zero or byte one. Block 962 also sends the EOFLK commandto the IOC on the system bus RDDT lines thereby informing the IOC thatthe address is on the system bus and also terminating the CPU-IOC linkestablished above in block 952. The CPU firmware then exits to block 963to output the range which in turn enters block 947 via block 950.

When block 947 is entered the second time, to output the range to theIOC, the channel number and function code are again sent to the IOCalong with the CPCMD command. When the addressed IOC responds withproceed, block 951 is entered followed by block 952 which reestablishesthe CPU-IOC link for the second time. Block 953 is then entered and thefunction code tested, this time the output range function code OD (hex)will result in block 964 being entered. In block 964 the PDMCIOflip-flop is tested to determine the IOC type and if a DMC IOCresponded, block 965 will be entered. In block 965 the CPU firmware getsthe range from the SPM work location and stores the range into thesecond word of the program channel table entry associated with thechannel number (see FIG. 30). Following block 965 if a DMC IOC respondedor if a DMA IOC responded, block 966 is entered and a test is made todetermine whether the range specified in the IOLD software instructionis equal to zero. If the range is zero, block 967 is entered and the CPUsends an EOFRG command to the IOC on the system bus RDDT lines. Some I/Ocontrollers store this end-of-range condition in their status indicatorsand other IOCs ignore this condition. In block 968 the CPU sends therange, in number of bytes, to the IOC on the system bus address/datalines (see FIG. 17, data word format) and also sends the EOFLK commandon the system bus RDDT lines. DMA and some DMC IOCs store the rangewithin the IOC and other DMC IOCs ignore the range. The EOFLK command inblock 968 terminates the CPU-IOC link established in block 952 and freesthe system bus for other use. The termination of the CPU-IOC linkcompletes the processing of the IOLD software instruction and the CPUfirmware exits to fetch the next software instruction in block 969.

Before leaving the discussion of the IOLD software instruction it shouldbe noted that, from the software point of view, the results of executingthe software IOLD instruction are the same whether or not the I/Ocontroller is a DMA I/O controller or a DMC I/O controller. Thedifferences between the DMA and the DMC I/O controllers are masked fromthe software by the CPU firmware. Again as in the case of the IOsoftware instruction, the programmer need not be cognizant of whetherthe device for which he is writing an input/output program is attachedto a DMA or DMC I/O controller.

Although the software programmer need not be cognizant of the IOC type,there is a difference in the way the system responds to an IOLDinstruction which specifies an out of bound I/O buffer depending onwhether the device is connected to a DMA IOC or DMC IOC. As describedabove, for a DMC I/O controller, the IOLD CPU firmware checks whetherthe end of the I/O buffer is within the physical memory present in thesystem and if not an unavailable resource (trap 15) will result and theexecution of the IOLD software instruction will be terminated withoutever initiating any data transfer between the IOC and the main memory.Because main memory within the system must be physically contiguous(i.e., no holes in the address space), if the end main memory locationof the IO buffer is physically present in the system, the beginning andall in between locations must also be present. Therefore, this initialcheck for the DMC I/O controllers alleviates the necessity of checkingwhether each individual location is physically present in the system ona per DMC data transfer operation. For DMA I/O controllers, no initialI/O buffer range check is made and a check is made during each DMA datatransfer to determine whether the addressed location is physicallypresent in main memory. Therefore, DMA I/O controllers must be capableof storing the unavailable resource indicator received on system buslines MEMPER or PMMPAR and storing this error indicator for laterreporting to the CPU when the status of the transfer is requested by theCPU.

IOH Instruction

The IOH software instruction is used to send or receive control bytes ordata to/from I/O controllers. This instruction is similar to the IOinstruction except that it deals with a byte transfer instead of a wordtransfer.

As with the IO software instruction, CPU firmware controls execution ofthe IOH instruction. The flow chart of the IOH instruction is similar tothat of the IO instruction with the following differences. Referring nowto FIG. 28, for an IOH instruction: in block 904 a byte address isdetermined; in block 927 a byte of data is accessed; in block 928 a byteis sent to the IOC; and in block 933 a byte is read from the IOC. Systembus operations during IOH instruction execution are described above.

Traps and Software Interrupts

Software interrupts are caused by events external to the currentinstruction being executed by the CPU, such as power failure, IOCinterrupt, etc., which are acted upon at the completion of the currentsoftware instruction. Traps, however, are caused by events related tothe current software instruction, such as parity error, program error,addressing nonexistent resource, etc., and are acted upon immediately,not waiting for instruction completion.

Software Interrupts

Every program in the central processor executes at some softwarepriority level but can be interrupted by an event with a higherpriority. Each interrupting event is assigned a priority level. Thereare 64 levels of software interrupts, numbered from 0 to 63; level 0 hasthe highest priority, 63 the lowest. Priority levels 0, 1 and 2 arereserved for fixed events, other events are dynamically assignedinterrupt levels by software. In the preferred embodiment, the 64software interrupt levels are assigned as follows; power failure islevel 0 (highest level), watchdog timer (WDT) runout is level 1, usedlast trap save area is level 2, real time clock (RTC) is any level otherthan 0-2 (software assigned level is indicated in main memory location0016 hex), peripheral device or IOC requiring service is any level otherthan 0-2 (dynamically controlled by software), and the level change(LEV) software instruction--any level (the level is specified in the LEVsoftware instructions).

Associated with each software interrupt level is a dedicated main memorylocation that contains an interrupt vector. The interrupt vector is apointer to an interrupt save area (see FIG. 31) that is associated withthe level. Software sets up an interrupt save area for each level activein a particular software program. An interrupt save area always containssix locations and can contain an additional sixteen. The layout of eachinterrupt save area is as follows:

The first location is a pointer to a list of Trap Save Areas (TSAP)currently associated with this level.

The second location contains the channel number and interrupt level ofthe interrupting device (DEV). This location is loaded by CPU firmware.

The third location contains the interrupt save mask (ISM). This maskdetermines which registers are to be saved in the variable locations.

The fourth location is reserved for future use and must be zero (MBZ).

The fifth location contains the interrupt handler procedure (IHP)pointer. It acts as a pointer to the interrupt handling procedure(software program) for a new level and is used to store the returnaddress for the return from the interrupt handling procedure.

The sixth location stores the status (S) register (i.e., interrupt leveland CPU ID).

The remaining locations are reserved for the registers stored undercontrol of the interrupt save mask. If the mask is all zeros none ofthese locations is used.

An active/inactive flag bit for each interrupt priority level ismaintained in dedicated main memory (see FIG. 32). Flags are set when asoftware instruction is initiated and are set/reset by a software levelchange (LEV) instruction. CPU firmware scans these flags to determinethe highest level software interrupt to be processed.

Traps

Traps are caused by events that are synchronous with the execution ofthe current software instruction. Associated with each type of trap is adedicated main memory location containing a pointer to the software traphandling procedure. These locations are called trap vectors (TV).Seventeen trap vectors are available (location 066F through 007F in FIG.32), in the preferred embodiment not all are used. The preferredembodiment has trap vectors that are used to handle the following eventswith the trap vector number listed in parenthesis: parity error (17),program error (16), unavailable resource (15), privilege operationviolation (13), integer arithmetic overflow (6), uninstalled (nonscientific) operation (5), uninstalled scientific operation (3),trace/break point trap (2), and monitor call (1).

When a trap event occurs, CPU firmware aborts execution of the softwareinstruction causing the trap and extracts the trap handling procedurepointer from the associated trap vector in main memory and branches tothe trap handler. A trap can occur at any software interrupt level andseveral traps can be pending at one time. A trap could be entered at onesoftware level, that level interrupted during execution of the softwaretrap procedure, and then the same software trap procedure entered at adifferent level, or a new trap could occur while processing the originaltrap.

To accommodate this possibility, a pool of trap save areas is available.These trap save areas are maintained in main memory and are used tostore certain registers and information related to that trap (see FIG.31). Dedicated main memory location 0010 (head) always points to thenext available trap save area. When a trap occurs, CPU firmware storesthe context of the related registers in the next available trap savearea and the pointer (TSAP) in the first word of the current interruptsave area is adjusted so that it points to the trap save area (i.e., thenew trap save area is linked at the beginning of the list). The pointerin the first location of the trap save area link (TSAL) is a linkpointing to any other traps that occurred at the same interrupt level.If this location is null in the list (zero), it indicates that this areais the last trap save area for the software interrupt level. If the linkis not null, it points to the next trap save area associated with thissoftware interrupt level. In turn, the first location of the pointed totrap save area may be null or point to another trap save area for thatsoftware interrupt level. At the end of each trap handling procedure, areturn from trap software instruction must be executed; this causes arestoration of the trap context that was saved, unlinks the trap savearea from the beginning of the list and resets the link pointer to itsoriginal state.

The relationship of traps and interrupts and their vector linkage isshown in FIG. 31. The trap vector points to a trap handling procedure.The trap save areas are associated with an interrupt level. Interruptvectors point to interrupt save areas which in turn point to associatedtrap save areas and interrupt handling procedures. The trap save areascontain the following information:

The first location contains the trap save link (TSAL) that points to anyother trap save areas associated with this interrupt level.

The second location stores the contents of the indicator (I) registerwhen a trap occurs.

The third location stores the contents of general register R3 when atrap occurred.

The fourth location stores the first word of the software instruction(INST) causing the trap.

The fifth location (Z) stores miscellaneous information (i.e., size oftrapped instructions, a field information valid, privilege state, etc.).

The sixth location (A) stores the effective address generated by thetrapped software instruction.

The seventh location stores the program (P) counter address used toreturn from the trap handling procedure.

The eighth location stores the contents of the base (B3) register whenthe trap occurs.

FIRMWARE OVERVIEW GENERAL DESCRIPTION OF FIRMWARE FLOW

The CPU firmware comprises a set of functional routines that areresident in the 1024 word by 48-bit read only store (ROS) (element 238in FIG. 9). These routines control various hardware operations inresponse to software instructions and hardware conditions. Theseoperations are: initialization/power-up autostart, instructionextraction, instruction execution, and interrupts (hardware orsoftware).

When a firmware step is decoded, certain hardware operations occur suchas causing the ALU to arithmetically add the contents of two registersand load the register contents back into one of the registers or writethe request into scratch pad memory.

Sequencing is performed by grouping microoperations intomicroinstructions, and then, by grouping microinstructions. Onemicroinstruction is performed during each firmware step (CPU cycle) of500 nanoseconds. These sequences of microinstructions are termedmicroroutines or microprograms. They provide a link between softwarecontrolled system programming and hardware operation.

During software instruction extraction from main memory which is doneunder CPU firmware control, branching is performed, on softwareinterrupts and the format of the software instruction to be executed, toselect a particular microroutine in the ROS containing the firmware forprocessing a pending software interrupt or the software instructionwhich was just read from main memory. As each firmware instruction(microinstruction) of the microroutine is decoded, it in turn enablesthe proper hardware paths.

After the specified microinstruction is performed, either the nextfirmware microinstruction is addressed and executed or a branched-tomicroinstruction is executed. Under some circumstances a branch oncondition test is performed to determine the next microinstructionaddress.

In this way, the CPU firmware cycles through various sequences requiredto complete the execution of the software instruction. When complete,the next software instruction is fetched from main memory and executedin similar fashion.

The general firmware flow is shown in FIG. 33. The system is initializedby either applying power to the system, block 350, or depressing theCLEAR pushbutton on the control panel, block 352. The initializesequence, block 354, runs the firmware resident quality logic test (QLT)and bootloads software into main memory from a peripheral deviceattached to the system bus. When the bootload is completed, the firmwareloop for software instruction extraction, block 356, and execution,block 358, is entered. If a trap condition occurs, the trap firmware isentered to process the condition. The trap routine, block 362, firmwaredoes the initial processing of the trap condition and exits to thepriority level change routine, block 374. In the priority level changeroutine, no software priority level change is made (i.e., the trap isprocessed on the same software priority level as the level of thesoftware instruction causing the trap) but the firmware completes theprocessing of the trap save area and sets up the program counter sothat, upon exiting to the software instruction extraction sequence, thefirst software instruction of the trap handling procedure will beextracted. The software trap handling procedure will terminate byexecuting a return from trap (RTT) software instruction which restoresthe registers that were saved in the trap save area and control returnsto the next software instruction to be executed (determined by the eventthat caused the trap). It should be noted that the trap routine block362 of FIG. 33 is a set of CPU firmware microinstructions which areexecuted in preparation of the CPU starting the execution of thesoftware trap handling procedure shown in FIG. 31.

Referring again to FIG. 33, any servicing of an I/O software interrupt,block 360; the watch dog timer, block 364; or real time clock, block364; is performed by firmware at the completion of the softwareinstruction extraction sequence and before the software instructionexecution sequence is begun. Upon completion of the interrupt firmware(blocks 360 or 364), the firmware branches back to the softwareinstruction extraction sequence.

If the software priority level of the software interrupt is greater thanthe priority level of the program currently being executed by the CPU asdetermined by the firmware in block 360, or if a counter associated withthe watch dog timer (WDT) or real time clock (RTC) in block 364 isdecremented to zero, the priority level change firmware routine block374 is entered and the currently executing software program isinterrupted. In the case of a priority level change, the priority levelchange routine exits to the software instruction extraction sequence,block 356, and begins executing the first software instruction of theinterrupt handling procedure associated with the software interruptbeing serviced. Upon completion of the software interrupt handlingprocedure, the CPU will change software priority levels by executing asoftware level change (LEV) instruction. If the interrupted program isthe highest priority level waiting to be executed, the firmware willpickup execution of the interrupted software program be reextracting thesoftware instruction that was interrupted between extraction andexecution. If the software priority level of the software interrupt isless than or equal to the priority level (again as determined by thefirmware a block 360) of the program currently being executed by theCPU, software priority levels will not be changed and interrupt routineblock 360 will exit by branching back to the software instructionextraction sequence in block 356 which will result in the reextractionof the software instruction whose execution was aborted. In this casethe lower priority software interrupts will remain pending and will behonored when the CPU lowers the priority level of the software programbeing executed by eventually entering the priority level change routineblock 374. It should be noted that the software interrupt routine block360 and priority level change routine 374 of FIG. 33 are a set of CPUfirmware microinstructions which are executed in preparation of the CPUstarting the execution of the software interrupt handling procedureshown in FIG. 31.

Referring again to FIG. 33, if a hardware interrupt condition isdetected, an immediate forced entry, block 368, to the hardwareinterrupt firmware, block 370, is executed and the condition isserviced. At the completion of the hardware interrupt sequence a returnbranch, block 372, to the interrupted firmware flow is executed.SOFTWARE INTERRUPT, TRAP, HARDWARE INTERRUPT INTERACTION

The relationship of the software program being executed by the CPU andthe CPU firmware can be seen in FIG. 34 which shows the softwareinterrupt, trap, and hardware interrupt interaction.

Software Program

Now referring to FIG. 34, the current software (SW) program beingexecuted in the CPU is shown as block 380. In the preferred embodiment,during execution the current software program 380 is resident in mainmemory and one instruction at a time is read from main memory andprocessed by the CPU under firmware control. In FIG. 34, three softwareinstructions are shown in detail, the previous software instruction381S, the current software instruction 382S and the next softwareinstruction 383C. The modifiers: previous, current, and next, deal withthe temporal relationship of the software instructions and need notnecessarily describe the spatial relationship of the softwareinstructions. That is, in practice the previous software instruction maybe located at location 1000 in main memory and if it is a branchinstruction it may branch to the current software instruction located atlocation 2000. with the next software instruction located at location2001. Therefore, the modifiers: previous, current, and next, describethe order in which the instructions are executed and not necessarilytheir relative location within main memory.

Firmware Microprograms

Again referring to FIG. 34, previous software instruction firmware block381F, current software instruction firmware block 382F and next softwareinstruction firmware block 383F represent the sequence of CPU firmwaremicroinstructions which are executed in processing previous softwareinstruction 381S, current software instruction 382S and next softwareinstruction 383S respectively. It should be noted that the firmwarerepresented by blocks 381F through 383F represent the sequence of CPUfirmware microinstructions which are executed and not necessarily thefirmware microinstructions themselves which are individually stored inthe firmware ROS (238 in FIG. 9). It should be noted that the firmwareinstructions executed in block 382F can be the same as the firmwareinstructions executed in block 381F if the current software instructions382S is the same as the previous software instruction 381S. Inparticular it should be noted that the extraction firmware block 384within the current software instruction firmware block 382F is alsoexecuted in the previous software instruction firmware block 381F andthe next software instruction firmware block 383F as is softwareinterrupt decision block 385, both of which are independent of theparticular software instruction being executed. As with the softwareinstruction relationship, the terms previous, current and next withregard to the software instruction firmware show the temporalrelationship between the firmware execution sequences and do notrepresent the spatial relationship of the firmware instructionsthemselves as stored within the firmware ROS. Thus it can be seen inFIG. 34 that when the previous software instruction firmware 381Fcompletes the processing of the previous software instruction 381S thecurrent software instruction firmware 382F is entered and upon itscompletion the next software instruction firmware 383F will be entered.

Software Interrupts, Hardware Interrupts and Traps

Referring now to current software instructions firmware block 382F, itcan be seen that the execution of the current software instruction 382Smay be broken by the occurrence of a software interrupt, a hardwareinterrupt, or a trap. As will be seen below, the occurrence of asoftware interrupt will always result in a suspension of the processingof the current software instruction and may, depending upon the softwareinterrupt priority level, result in the execution of the softwareinterrupt handler procedure prior to recommencing the execution of thecurrent software instruction. The occurrence of a hardware interruptresults in the suspension of the execution of the current softwareinstruction firmware during which time the CPU firmware processes thehardware interrupt and upon its completion returns to the processing ofthe current software instruction at the point of the hardware interrupt.The occurrence of a trap will result in the abandoning of the executionof the current software instruction and the processing of the trap by atrap handler procedure which is a set of software instructions dedicatedto processing that trap. Upon completion of the trap handler proceduresoftware, the next software instruction following the trapped softwareinstruction may be processed or another software program execution maybe started as will be seen below. Software Interrupts

Now, turning to the discussion of the current software instructionfirmware 382F in detail, the processing of the current softwareinstruction by the CPU firmware is begun by the firmware extracting thecurrent software instruction from main memory under firmware control inblock 384. Upon completion of extracting the first word of the currentsoftware instruction, a firmware branch is made to test if there is anysoftware interrupt pending in block 385. If there are one or moresoftware interrupts pending, the firmware branch branches to thefirmware microprogram of the highest priority pending softwareinterrupt. In FIG. 34 there are two software interrupt firmware routinesshown, blocks 388-1 and 388-2 with each block representing a CPUfirmware microprogram to handle a particular software interrupt. Withina particular software interrupt firmware microprogram, such as 388-2, atest is made to see whether the priority of the software interrupt ishigher than the priority of the currently executing software program.This test is done in block 389. If the software interrupt is of lower orequal priority to that of the currently executing software program, theinterrupt will not be accepted and the low or equal branch from block389 is taken and the firmware recommences the execution of the currentsoftware instruction by reentering block 382F followed by thereextraction of the current software instruction from the main memory inblock 384. If the priority of the software interrupt is higher than thecurrently executing software program, the software interrupt is acceptedand block 389 enters block 390 which sets up the interrupt save areawhich saves the state of the current software program and branches tothe software interrupt handling procedure associated with the softwareinterrupt. The software interrupt handling procedure, which is a set ofsoftware instructions, in block 391 is then executed with each of thesoftware instructions contained therein being processed by the CPUfirmware. The last instruction within the software interrupt handlerprocedure software is a level change (LEV) software instruction 391Lwhich results in the CPU firmware scanning for the highest prioritysoftware program which is awaiting execution. The level softwareinstruction 391L is executed by the level instruction firmwaremicroprogram 392 which in block 393 determines the highest prioritysoftware program currently awaiting execution. If a higher prioritysoftware program 394 is awaiting execution, that software program isexecuted under CPU firmware control and is also terminated by an LEVsoftware instruction 394L. The level of software instruction 394Lresults in the execution of the LEV instruction firmware microprogram392 and again a test for the highest priority software program level isperformed in 393. If the level test in 393 determines that thepreviously suspended current software program 380 is the highestpriority program awaiting execution, the current software instructionfirmware 382F is entered and the current software instruction 382S isreextracted by extraction firmware 384 and the CPU firmware execution ofthe current software instruction continues.

In summary, it can be seen that the CPU firmware while executing asoftware instruction: tests for the pending of a software interrupt, isvectored to the particular software interrupt firmware microprogramwhich is dedicated to handling a particular software interrupt, that thesoftware interrupt firmware tests for the priority of the softwareinterrupt relative to the priority of the current software program, andthat if the priority is lower or equal to that of the current softwareprogram the current software program is not interrupted and theprocessing of the current software instruction by the CPU firmware isrecommenced. On the other hand, if the software interrupt is of higherpriority than the currently executing software program, the softwareinterrupt is serviced by saving the state of the current softwareprogram in the interrupt save area and commencing execution of thesoftware interrupt handler procedure which is itself a set of softwareinstructions. The software interrupt handling procedure then terminatesby a level software instruction which will sooner or later result in thereactivation of the interrupted current software program with theexecution of the current software instruction being reinstituted byreextracting the current software instruction from main memory.

Hardware Interrupts

If, during the execution of the current software instruction there areno software interrupts pending, block 385 will exit to the executionfirmware block 386. The execution firmware, block 386, performs thosefirmware steps necessary to complete the execution of the currentsoftware instruction. During the course of the performing of theexecution firmware in block 386, a hardware interrupt may occur. Unlikesoftware interrupts which are detected by the CPU firmware branching onthe pending of the software interrupt to a particular software interruptfirmware routine to handle the particular pending software interrupt,hardware interrupts are not tested for by the CPU firmware but insteadresult from the hardware interrupt logic forcing the CPU firmware tobegin executing a sequence of microinstructions associated with thehardware interrupt. In FIG. 34, there are four hardware interruptfirmware microprograms shown, 395-1 through 395-4. These hardwareinterrupt firmware microprograms 395-1 through 395-4 are composed of CPUfirmware microinstructions which are designed to handle the particularinterrupt condition which caused the hardware interrupt. Of these fourhardware firmware microprograms, two are for non-trap hardware interruptconditions and are shown as 395-1 and 395-2. Looking at non-trapcondition hardware interrupt firmware routine 395-2 it is seen that thelast microinstruction, 395-2R, of the microprogram contains a hardwareinterrupt return microoperation which results in the firmware returningcontrol to the firmware microinstruction following the one that wasexecuted just prior to the occurrence of the hardware interrupt. Forexample, a condition which can result in a hardware interrupt is theoccurrence of a DMA data transfer request on the system buses. In thiscase the execution of the current software instruction by the CPUfirmware is suspended, the DMA data transfer is handled by CPU firmwareroutine and upon its completion, the execution of the current softwareinstruction is picked up at the point of occurrence of the DMA hardwareinterrupt. Returning now to the two trap condition hardware interruptfirmware microprogram shown in FIG. 34, blocks 395-3 and 395-4, theoccurrence of a hardware interrupt associated with a trap condition willcause the associated hardware interrupt firmware microprogram to beentered, such as 395-4. The trap condition hardware interrupt firmwaremicroprogram does some preliminary processing of the hardware interruptprior to exiting to a trap firmware microprogram which completes theprocessing of the trap. For example, block 395-4 exits to trap firmwareblock 396-2. Because the trap condition hardware interrupt firmwaremicroprograms, 395-3 and 395-4, do not return to the execution firmware386 at the point of interruption, the execution of the current softwareinstruction is aborted in the case of the trap condition being detectedby a hardware interrupt. On the other hand, the non-trap conditionhardware interrupts result only in the suspension of the execution ofthe current software instruction because they return to the executionfirmware 386 at the point of interruption. An example of the trapcondition which is detected by a hardware interrupt is a main memoryparity error which will result in the occurrence of a hardwareinterrupt. In case of parity error, the CPU firmware is not returned toprocessing the current software instruction at the point of theoccurrence of the hardware interrupt but instead exits to a trapfirmware microprogram such as 396-2 to continue processing the parityerror. This results in the aborting of the execution of the currentsoftware instruction. As can be seen in FIG. 34, vectored hardwareinterrupts can occur only during the execution firmware block 386 andthen only during the time in which hardware interrupts are enabled(i.e., not inhibited). As can be further seen in FIG. 34, during theprocessing of a hardware interrupt, the hardware interrupt firmwaremicroprogram itself inhibits hardware interrupts such that the hardwareinterrupt firmware microprograms will not themselves be interrupted by afurther hardware interrupt (i.e., hardware interrupts are not nestedwhereas software interrupts can be nested). Traps

Now returning to the execution firmware block 386, it can be seen thatduring the execution of the current software instruction the firmwareitself may conduct one or more tests for trap conditions. For example,block 387 represents a CPU firmware test for the existence of a trapcondition and if a trap condition is detected, the execution firmwarebranches to trap firmware block 396-3 to process the trap condition.Later during the execution of the current software instruction one ormore other trap tests may be conducted by the firmware. One more traptest is shown as block 388 which, if it detects a trap condition, willexit to trap firmware microprogram 396-1. Turning now to trap firmwaremicroprogram 396-3, the function of the trap firmware microprogram is toset up the trap save area in which the state of the current softwareprogram is saved before the CPU firmware begins processing the traphandler procedure. The trap handler procedure is a set of softwareinstructions written to process the trap condition. The trap handlerprocedure 397 is associated with the particular trap condition detectedby block 387 and further processed by block 396-3. There is a separatetrap handler procedure for each of the trap conditions detected by theCPU firmware. The trap handler procedure 397 is then executed by the CPUfirmware, as is any other software program, and terminated by a returnfrom trap (RTT) software instruction 397R. The return from trap softwareinstruction 397R is executed by RTT instruction firmware 398 which mayrestore the software context saved in the trap save area by trap 396-3and return to begin processing the next software instruction by enteringthe next software instruction firmware block 383F. Alternatively, RTTinstruction firmware 398 may result in the starting of the execution ofanother software program as shown in block 399. The exit from the RTTinstruction firmware block 398 is determined by the contents of the trapsave area which may have been modified by the trap handler softwareprocedure 397 during the processing of the trap condition. An example ofa condition which may be handled by a trap is the detection by theexecution firmware in block 387 of a scientific software instructionoperation (floating point) which will result in trap firmware block396-3 being entered. In block 396-3 the trap save area is set up andblock 397 is entered. Trap handler procedure 397 will be a set ofsoftware instructions to simulate the results of these optionalscientific software instructions and will perform the indicatedoperation. The execution of the return from trap software instruction379R will cause the RTT instruction firmware 398 to be executed which inturn will result in the next software instruction firmware 383F to beexecuted thereby executing the next software instruction 383S. It can beseen that in this example the occurrence of a trap results in theaborting of the execution of the current software instruction and thecompletion of the current software instruction by the trap handlerprocedure software routine followed by the execution of the nextsoftware instruction.

Interrupts and Traps

In summary it can be seen that the software interrupts and traps occurby the CPU firmware making explicit firmware tests for variousconditions during the execution of a software instruction. The softwareinterrupt will further result in the recommencing of the execution ofthe current software instruction, starting with the reextraction of theinterrupted current software instruction from main memory. On the otherhand, a trap will result in the aborting of the execution of the currentsoftware instruction and, depending upon the trap handler proceduresoftware written to handle the particular trap condition, may or may notresult in the execution of the next software instruction. Hardwareinterrupt conditions need not be tested for by the CPU firmware. Theoccurrence of a hardware interrupt results in suspension of theprocessing of the current software instruction followed by the servicingof the hardware interrupt by the CPU firmware. If the hardware interruptis for a non-trap condition, the hardware interrupt firmwaremicroprogram then returns to the processing of the current softwareinstruction at the point of interruption. If the hardware interrupt isassociated with a trap condition, a trap firmware routine is entered andthe return to the processing of the current software program isdetermined by the particular trap handler procedure software programassociated with the trap condition. If the trap handler procedure doesreturn control to the current software program, control is usuallyreturned such that the next software instruction will be processed.

CPU FIRMWARE WORD DESCRIPTION

The overall CPU firmware word is shown in FIG. 35. The microinstructionword is partitioned into four major fields with major fields subdividedinto various subfields.

The scratch pad memory control field of the CPU firmware word is shownin FIG. 35A.

Bits 0 through 7 are used to control both the scratch pad memory (SPM,element 236 in FIG. 8) and the microprocessor register file (element 268in FIG. 8). The subfields are shown in FIG. 35A. Bit 0 determines thescratch pad memory operation. When set, a binary ONE, data can bewritten and when reset, a binary ZERO, data can be read. Bits 1, 5, 6and 7 comprise the scratch pad memory work location addresses, locations00 through 0F in FIG. 10, to/from which data is being written or read.Bits 2, 3 and 4 address registers in the microprocessor RAM (registerfile 268 in FIG. 8).

The arithmetic logic unit (ALU) control field of the CPU firmware wordis shown in FIG. 35B.

Bits 8 through 19 are used for ALU (element 266 in FIG. 8) control. Thesubfields are shown in FIG. 35B.

Bits 8 and 9 provide a shift type control while bits 10, 11 and 12determine the source of the data to be processed. Bits 13, 14 and 15determine the function to be performed on that data by themicroprocessor ALU. Bit 19 is set if a carry inject is required for theoperation. The carry in, in turn, is used to modify the source andfunction combination in accordance with FIG. 36.

Bits 16, 17 and 18 designate the destination, in accordance with FIG.36, to which the data, resulting from the function performed by the ALU,is to be sent.

Bits 8 and 9 in addition to providing shift type control for the ALUalso serve as main memory read/write control when bit 23 is a binary ONEindicating that a main memory operation is to be performed.

The subcommand and control field of the CPU firmware word is shown inFIG. 35C.

Bits 20 through 35 comprise a subcommand and control field. Thesubfields are shown in FIG. 35C.

Bits 20 and 21 control the input ports on the data selector multiplexer(269 in FIG. 8), thereby determining the source of data sent to the ALU.Bit 23 is a memory control bit which when set, a binary ONE, causes amemory go (MEMGO) signal on system bus B to initiate a main memory read,write or refresh cycle. The memory operation to be performed isdetermined by bits 8 and 9 of the microinstruction. The functions ofbits 24 through 31 are determined by the states in bits 32 through 34which are used as a subcommand decode field.

Bit 35 controls hardware interrupts; when set, a binary ONE, it enableshardware interrupts and when reset, a binary ZERO, it inhibits hardwareinterrupts.

FIG. 37A lists the various interrupt commands that can be decoded whensubcommand decode bits 32 through 34 equal a binary 110. When firmwarebits 32-34 indicate an interrupt command, bits 24 through 27 are ignored(don't care). FIGS. 37B through 37E list other commands that can bedecoded when the subcommand decode bits 32 through 34 equal a binary101. When subcommand decode bits 32-34 equal 101 (binary), the controlpanel commands listed in FIG. 37B are performed if the bits 24-27 equal3 (hex), which as shown in FIG. 37C will result in a control panelstrobe (bits 24-26 equal 001, bit 27 equal don't care). When subcommanddecode bits 32-34 equal 101 (binary) and bits 24-27 do not equal 0011(binary), coded combinations of the I/O commands listed in FIGS. 37Cthrough 37E are performed. These I/O commands are partitioned into setsof 3, 2 and 3 bits, allowing one command from each of these command setsto be executed simultaneously. Use of the various I/O commands listed inFIGS. 37C through 37E can be seen in FIGS. 20 through 23 whichillustrate the various sequences that occur on the system buses underCPU firmware control.

When bit 32 of the subcommand decode field is zero, the entiresubcommands field is interpreted as two subcommand fields, subcommandfield 1 (FIG. 37F) being specified by bits 24 through 27 of the firmwareword, and subcommand field 2 (FIG. 37G) being specified by bits 28through 31 of the CPU firmware word. In addition, when bit 32 is a zero,bits 33 and 34 provide main memory control as shown in FIG. 35C.

The read only storage (ROS) addressing field of the CPU firmware word isshown in FIG. 35D. Bits 36 through 47 comprise the ROS addressing field.The subfields are shown in FIG. 35D. The ROS addressing field determinesthe next sequential firmware address used to access the ROS (element 238in FIG. 9). Bits 36 and 37 determine the type of address found in setsof bits between 38 and 47 as listed in FIG. 35D. When an unconditionalbranch is indicated (a binary 00 in bits 36 and 37), the firmware willbranch to the address contained within bits 38 to 47. When a branch ontest is indicated (a binary 01 in bits 36 and 37), bits 38 through 41and bit 47 are used, indicating a two-way branch. When a branch multipletest is indicated (a binary 10 in bits 36 and 37), bits 40-43 are used,indicating a 16-way test branch. When bits 36 and 37 are both set, theaddress contained in bits 38 through 47 is ignored and the firmwareaddress to which the flow returns after the hardware interrupt isobtained from the hardware interrupt return address register (252 inFIG. 9).

SCRATCH PAD MEMORY CONTROL

The Scratch Pad Memory Control Field of the CPU firmware word is shownin FIG. 35A. Bits 0, 1 and 5 through 7 are used to control the ScratchPad Memory (SPM) (element 236 in FIG. 8). Bit 0 determines the scratchpad memory operation. When set, a binary ONE, data can be written andwhen reset, a binary ZERO, data can be read. Bits 1 and 5 through 7 formthe address used to address the scratch pad memory work locations,locations 00 through 0F in FIG. 10, to/from which data is to be writtenor read. Bits 2 and 3 control the selection of the 3 low order bits ofthe 4-bit address used to address the microprocessor random accessmemory (RAM) (register file 268 in FIG. 8). The various combinations ofbits 2 and 3 permit the low order bits of the microprocessor RAM addressto be selected from the subfields (FR0, FR2 and FR3) of the function (F)register (274 in FIG. 8) or from bits 5 through 7 of the firmware worditself. The high order bit (bit 0) of the microprocessor RAM address isalways determined by bit 4 of the firmware word. This ability of the CPUfirmware word bits 2 and 3 to control the selection of themicroprocessor RAM addressing bits from the various subfields of the Fregister is important in allowing for the fast decoding of the softwareinstruction contained in the F register. Within a software instruction,subfield FR0 in the F register usually contains a register number andsubfields FR2 and FR3 compose the address syllable of the softwareinstruction.

ARITHMETIC LOGIC UNIT CONTROL

The ALU control field of the CPU firmware word is shown in FIG. 35B.Bits 8 through 19 are used for microprocessor (element 232 in FIG. 8)control. The subfields are shown in FIG. 35B. As indicated hereinbefore,the 16-bit microprocessor 232 in FIG. 8 is composed of cascading four4-bit sliced microprocessors of the type Am2901 microprocessor producedby Advanced Micro Devices Inc., of Sunnyvale, Calif. Some of themicroprocessor control fields are used directly by the cascaded bitsliced microprocessors and others (shift type control and carry inject)are used to control the end conditions generated by the most significantbit microprocessor and the least significant bit microprocessor.

Firmware word bits 8 and 9 provide shift type control by controlling thebits shifted into and out of the most significant and least significantbits of the cascaded microprocessors. Bits 10 through 12 determine themicroprocessor ALU source and are used to directly control each of thefour bit sliced microprocessors. Exact definition of these bits may befound in the publication "A Micro Programmed 16-Bit Computer" publishedby Advanced Micro Devices, Inc., of Sunnyvale, Calif., and incorporatedherein by reference. Bits 13 through 15 control the microprocessor ALUfunction and are also used directly by each of the 4-bit slicedmicroprocessors. Bits 16 through 18 control the microprocessordestination and again are used directly by each of the four bit slicedmicroprocessors. Bit 19 controls the carry inject input and is useddirectly by the least significant of the four cascaded bit slicedmicroprocessors. The microprocessor ALU source subfield controls thesource of the ALU inputs and may select between the primary (A) outputof the microprocessor register file, the duplicate (B) output of themicroprocessor register file, the output of the microprocessor internalwork register (Q), or data (D) input from the scratch pad memory. Themicroprocessor ALU function subfield determines the operation (e.g.,arithmetic addition, logical AND, etc.) to be performed by thearithmetic logic unit. The microprocessor destination subfielddetermines the destination to which the data resulting from the functionperformed by the ALU is to be sent. The operation performed by the16-bit microprocessor is a function of the microprocessor ALU source,microprocessor ALU function and carry inject subfields are shown in FIG.36.

In addition to providing shift type control, firmware bits 8 and 9 alsoprovide main memory read/write control. When firmware bit 23 is a binaryONE, bits 8 and 9 control whether a word, byte 0, or byte 1 is to bewritten into main memory or whether a word is to be read from mainmemory.

SUBCOMMANDS AND CONTROL

The subcommands and control field of the CPU firmware word are shown inFIG. 35C. Bits 20 through 35 comprise a subcommands and control field.Bits 20 and 21 control the data selector 269 of FIG. 8 which is afour-to-one multiplexer, the output of which is connected directly tothe data input ports of the microprocessor thereby determining thesource of data sent to the microprocessor ALU. The ALU input may beselected from; the SPM data register which contains the output of thescratch pad memory 236, the indicator (I) register 270 and M1 register272, a constant generated by using microinstruction bits 8, 9 and 24through 31 or the data from internal bus 260 (all shown in FIG. 8). Bit22 is scratch pad memory address select control and allows for themicroinstruction to address either the scratch pad memory work locations(locations 00 through 0F in FIG. 10) using microinstruction bits 1 and 5through 7 or the program channel table (locations 80 through FF) usingthe channel number obtained from the channel number register 296 in FIG.8. Bit 23 is the main memory go control bit and when set causes thePMEMGO- signal on system bus B to go low initiating a memory read/writecycle.

Bits 24 through 31 form a subcommands field, the meaning of which isinterpreted by bits 32 through 34 which form the subcommands decodesubfield. Depending upon the value in the subcommands decode subfield(bits 32 through 34), the subcommands field (bits 24 through 31) may beeither: an 8-bit constant, interrupt commands, control panel commands,I/O commands, function (F) register control or subcommands. The meaningof these various subcommands fields is tabulated in FIGS. 37A through37G.

Bit 35 is the hardware interrupt control field which permits the CPUmicroprogrammer writing firmware to inhibit or enable the occurrence ofhardware interrupts between microprogram firmware steps. When bit 35 isa binary ZERO, hardware interrupts are inhibited and the microprogramcannot be interrupted following the current microinstruction. When bit35 is a binary ONE, hardware interrupts are enabled and, if a hardwareinterrupt is currently pending, the microprogram will be interrupted atthe completion of the execution of the current microinstruction.

FIG. 37A lists the interrupt commands which can be decoded from bits 28through 31 (bits 24 through 27 are ignored). FIG. 37B lists the controlpanel commands which are decoded from bits 28 through 31 when bits 24through 27 are equal to 0011 (binary). The coded combinations of I/Ocommands listed FIGS. 37C through 37E are given in binary. These arepartitioned into sets of 3, 2 and 3 bits, allowing these sets of I/Ocommands to be executed simultaneously. When bit 32 is a binary ONE andbits 33 and 34 are binary ZERO's, the subcommands field is interpretedas function register control commands. When bit 32 is a binary ZERO, theentire subcommands field is interpreted as two subcommands, subcommands1 (see FIG. 37F) being specified by bits 24 through 27 of the firwareword and subcommands 2 (see FIG. 37G) being specified by bits 28 through31 of the CPU firmware word.

READ ONLY STORAGE ADDRESSING

The read only storage (ROS) addressing field of the CPU firmware word isshown in FIG. 35D. Bits 36 through 47 comprise the ROS addressing field.The subfields are shown in FIG. 35D. The ROS addressing field determinesthe next sequential firmware address used to access the next firmwareword from the ROS (element 238 in FIG. 9). Bits 36 and 37 determine thetype of branch address found in sets of bits between bits 38 through 47.

When bits 36 and 37 are 00 (binary), the firmware will branch to theaddress contained within bits 38 through 47 by using the 10-bit addressto retrieve the next firmware word from the ROS. When a 2-way testbranch is indicated (a binary 01 in bits 36 and 37), bits 38 through 41and bit 47 are used to select the test and generate the leastsignificant ROS address bit (bit 9). The four most significant bits(bits 0 through 3) of the next ROS address are taken from the four mostsignificant bits of the current ROS address and bits 42 through 46 ofthe firmware word are used directly as ROS address bits 4 through 8.These 2-way branches are used by the firmware to test the status ofvarious conditions such as the state of control flops 1 through 4(CF1-CF4) and various bits of the function register.

When a multiple test branch is indicated (a binary 10 in bits 36 and37), bits 38, 39 and 44 through 47 are used directly in the next ROSaddress and bits 40 through 43 are used to indicate one of sixteendifferent multiple test branches. The multiple test branches are used bythe firmware programmer to help decode the software instruction and torespond to software interrupts.

When bits 36 and 37 are both set (a binary 11), bits 38 through 47 ofthe firmware word are not used and the next ROS address is obtained fromthe hardware interrupt return address register (element 252 in FIG. 9)which contains the next ROS address at the time the CPU firmware wasinterrupted by a hardware interrupt. The hardware interrupt returnbranch is used by the firmware programmer at the end of a hardwareinterrupt firmware microprogram (block 395-2 in FIG. 34) to returncontrol to the point at which the firmware was interrupted by thehardware interrupt.

I/O CONTROLLER LOGIC DETAILS

Having described the operation of the central processor, the I/Ocontrollers and the various dialogs on the system buses, the I/Ocontroller logic will now be described in detail.

FIG. 38 is a logic block diagram of an I/O controller constructed inaccordance with the principles of the present invention. Referring toFIG. 38, it is seen that the major sections of the I/O controllerinclude: timing logic 400, DMA/DMC request logic 402, interrupt requestlogic 404, request reset logic 406, and device logic 407.

Timing logic 400 provides the basic I/O synchronization signals usedthroughout the I/O controller (signals PTIME3+20 and DMYTM3+). Inaddition, timing logic 400 takes the bus cycle out signal from theprevious I/O controller (or CPU if the I/O controller is the first I/Ocontroller on either system bus A or B) and delays the signal for 500nanoseconds before passing it on to the next I/O controller. Further,timing logic 400 is used to generate an IOC initialize signal(PCLEAR+20) which is used to initialize the controller and initiate theIOC's quality logic test (QLT) firmware.

The DMA/DMC request logic 402 is used to set the system bus DMA or DMCrequest lines (PDMARX- or PDMCRX-) to a binary ZERO during the IOC'stime slot if the IOC requires a DMA or DMC data transfer cycle and theDMA or DMC request line is not already set by another DMA or DMC I/Ocontroller on that particular system bus. A particular I/O controller iseither a DMA or DMC IOC. For DMA IOCs the DMA request line (PDMARX-) isused and for the DMC IOCs the DMC request line (PDMCRX-) is used.

Interrupt request logic 404 is used to set the bus interrupt requestline (PINTRX-) to a binary ZERO during the IOC's time slot if the IOCwishes to interrupt the execution of the software in the CPU and anotherIOC on the particular system bus has not already set the interruptrequest line. The IOC will initiate an interrupt sequence whenever astate change in one of the peripheral devices connected to the I/Ocontroller is sensed or whenever a particular I/O order is completed,for example, upon the expiration of the range following a read or awrite command.

The request reset logic 406 is used to reset the DMA or DMC or interruptrequest flops of the I/O controller in response to the CPU responseencoded on system bus lines RDDT 29+ through RDDT31+. As will be notedhereinafter, a given IOC can have both a DMA or DMC request and aninterrupt request pending concurrently. This is particularly true forIOCs which have multiple peripheral devices attached to them such thatone device may have completed a read or write operation, therebyrequesting an interrupt, and a second device may be in the process ofdoing a read or a write operation and be requesting the next word orbyte of data to be read or written to the peripheral device.

I/O CONTROLLER DEVICE LOGIC

Continuing to refer to FIG. 38, the operation of device logic 407 willbe discussed. Device logic 407 consists of command logic 409, task andconfiguration logic 429, interrupt logic 417, status and device ID logic437, data transfer logic 421 and address and range logic 445. It shouldbe noted that address and range logic 445 is only present in DMA I/Ocontrollers.

Command Logic

The command logic 409 decodes the I/O control commands and functioncodes addressed to the IOC. The command logic 409 determines: if the IOCcan accept the I/O command and sets line PROCED- to a binary ZERO; ifthe IOC is busy and sets line PBUSY- to a binary ZERO; or if the IOC istemporarily busy it sets both lines PROCED- and PBUSY- to binary ZEROsto inform the CPU to wait and retry the I/O command. The command logic409 determines the type of operation to be performed by the I/Ocontroller, generates a command cycle and if the I/O command isaccepted, enables a path from the system bus address/data lines to andfrom the I/O controller, or to the peripheral device. The command logicalso maintains the dialog link between the system bus and the IOC.Function code decoder 415 decodes the function code of the I/O commandcontrol word (see FIG. 24). Channel number switch 411 is set when thesystem is installed to contain the channel number of the I/O controller.Channel number comparator 413 compares the channel number set in channelnumber switch 411 with the channel number appearing on the systemaddress/data lines (BUSX00+ through BUSX08+) and if they are equal setssignal DMYCMD+ to a binary ONE. Signal DMYCMD+ is an input to requestreset logic 406. In addition, the channel number contained in channelnumber switch 411 may be transferred to the CPU via the system busaddress/data lines during a DMC data transfer request or during an I/Ointerrupt request sequence (see FIG. 17).

Only I/O commands with a channel number corresponding to the IOC'schannel number in channel number switch 411 are accepted by the IOC. Aswill be seen hereinafter, some of the I/O command decoding is done by adecoder in request reset logic 406. The command cycle is a sequence thatthe IOC performs while executing any type of input or output command(see FIG. 20). For input commands, the IOC reads stored controlinformation, status or device information and then transfers them ontothe system bus. For output commands, the IOC transfers controlinformation from the system bus into the device logic storage.

Task and Configuration Logic

During a command cycle, the task and configuration logic 429 is enabledafter the command logic decodes a function code that specifies thereading or writing of the task word, configuration word A orconfiguration word B. The meaning of individual bits in the task wordare device specific. The task word is intended for those functions whichhave to be output frequently as compared with relatively staticinformation which is output via the configuration word commands. Themeaning of individual bits in configuration words A and B are devicespecific. The configuration words are intended for those functions whichare output only infrequently. Configuration word B is used when moreinformation is required than can be coded into configuration word A. I/Ocontrollers always contain a task word register 431 whereas theexistence of configuration word A register 433 and configuration word Bregister 435 depend upon the amount of information required for aparticular peripheral device. Some peripheral devices require noconfiguration words while other peripheral devices require onlyconfiguration word A and still other peripheral devices requireconfiguration words A and B.

Interrupt Logic

During a command cycle, the interrupt logic 417 is enabled after thedecoded function code specifies the reading or writing of the interruptlevel contained in interrupt control word register 419. The interruptlogic is also used to generate an interrupt request when either of thefollowing conditions are present: the interrupt level is not equal tozero and the previous peripheral device operation is complete, or a stopI/O command is complete. When one of these conditions is present, aninterrupt cycle is initiated and the IOC sends an interrupt request tothe CPU by setting signal DAINOK+ to a binary ONE which is an input tointerrupt request logic 404. Setting of signal DAINOK+ to a binary ONEwill result in the setting of signal PINTRX- on the system bus to abinary ZERO. When the CPU acknowledges the interrupt request, the IOCloads the interrupt level and channel number onto the system busaddress/data lines for delivery to the CPU. The CPU then examines theinterrupt level. If the interrupt level is acknowledged by the CPUsetting line PROCED- to a binary ZERO, the interrupt operation iscomplete. If the interrupt level is not acknowledged by the CPU settingline PBUSY- to a binary ZERO, the I/O controller stacks the interruptrequest within the interrupt logic 417 and waits for the CPU to send aresume interrupt (RESUM) I/O command. When the CPU sends a resumeinterrupt I/O command, the IOC will reinitiate the interrupt request.

Status and Device Identification Logic

The status and device identification (ID) logic 437 is enabled by theIOC to store the status word(s) and device ID code. The status word 1,439, and status word 2, 441, contain peripheral device and main memoryconditions. The meanings of individual bits in the status words are IOCspecific. Status word 2 is only present in those I/O controllers whichhave more status information than can be coded into status word 1. Thedevice ID code is contained in device ID word 443 and represents thetype of peripheral device connected to the IOC. When the command logicdecodes an input status word 1 or 2 command, the status word 1 or 2 istransferred to the CPU on the system bus address/data lines. When aninput device ID command is decoded by the command logic, the device IDcode is transferred to the CPU via the system bus address/data lines(BUSX00+ through BUSX15+). The main memory error (parity or nonexistentaddress) signals MEMPER- (on system bus B) and PMMPAR- (on system bus A)are input by status and device ID logic 437 and are used to set theappropriate bit in status word 1, 439, for later reporting to the CPU.

Data Transfer Logic

After the data transfer has been initiated, the command logic 409enables the data transfer logic 421 for the transfer of data to or fromthe peripheral device. The IOC makes a data transfer request of the CPUby using signals DERSWT- and DMCINC- to cause DMA/DMC request logic 402to set signal PDMARX- (for DMA IOCs) or signal PDMCRX- (for DMC IOCs) toa binary ZERO.

If the IOC is a DMC IOC, a DMC request is sent to the CPU from the I/Ocontroller (see FIGS. 21A through 21D). The CPU acknowledges the requestand the IOC's channel number is loaded onto the system bus address/datalines for delivery to the CPU. After channel number delivery, a DMC datatransfer cycle is initiated and a byte of data is transferred by the CPUonto the system bus for the IOC (for output) or by the IOC onto thesystem bus for the CPU (for input). If output, a byte of data is takenfrom the system bus address/data lines and stored in data out register423 before it is delivered to the peripheral device. If input, a byte ofdata is taken from the peripheral device and held in data in register425 before being delivered to the CPU on the system bus address/datalines. In either case, for DMC I/O controllers, data byte align logic427 is not present. For DMC IOCs data byte alignment is done by the CPUas the data is transferred from the DMC IOC to the main memory or fromthe main memory to the DMC IOC.

Address and Range Logic

Address and range logic 445 is found only in DMA IOCs. In DMC IOCs thefunction performed by address and range logic 445 is performed by theCPU using the program channel table. Address register counter 447contains the 17-bit address output by the CPU during the output addressfunction of an IOLD software instruction. Address register counter 447is incremented as each word (or byte) of data is transferred between themain memory and the DMA IOC. Range register counter 449 is used to holdthe range (in number of bytes) of the data to be transferred. The rangecounter is initially set by the output range function of an IOLDsoftware instruction. As each word (or byte) of data is transferredbetween the DMA IOC and the main memory, the range register counter 449is decremented. The contents of address register counter 447 can betransferred to the CPU by an input address or input module I/O command.The contents of range register counter 449 can be input to the CPU by aninput range I/O command.

When the DMA IOC receives an acknowledge from the CPU in response to aDMA request, the controller is linked to the CPU for data transfer and aDMA cycle begins (see FIG. 22). Concurrent with the receipt of the CPUacknowledge, the address and range logic 445 activates the write byte 0and write byte 1 lines (PWRTB0+ and PWRTB1+). These two lines indicateto the main memory the type of read/write operation that is to beperformed. The address and range logic 445 loads the main memory addressonto the system bus address/data lines (BUSX00+ through BUSX15+) and theCPU activates the enable bus to CPU line (PENBSX-) to obtain it. If anonexistent main memory address error occurs, the CPU activates the mainmemory error lines (MEMPER- and PMMPAR-) during PENBSX- to notify theDMA IOC. When an input operation (write into main memory) is beingexecuted, signal PENBSX- remains a binary ZERO to transfer the data word(or byte) from data in register 425 onto the system bus and into themain memory. When an output operation (read from main memory) is beingexecuted, signal PENBSX- becomes a binary ONE. Upon receipt of anend-of-link command on system bus RDDT lines, the DMA IOC loads thecontents of data in register 425 onto system bus address/data lines (forinput) or unloads the system bus address/data lines into data outregister 423 (for output). If a main memory parity error occurs, the CPUnotifies the DMA IOC by activating signal MEMPER-/PMMPAR- during theend-of-link command. The DMA cycle is terminated and the data transfersequence is complete.

Note that data byte align logic 427 in data transfer logic 421 ispresent only for DMA IOCs. During DMA output operations, data byte alignlogic 427 works in conjunction with address and range logic 445 toextract the proper byte from the word of data received from the mainmemory (contained in data out register 423) and transfers the byte orword of data to the peripheral device. It should be noted that a word ofdata is always read from main memory. During DMA input operations it isthe function of the data byte align logic 427 to align the word (orbyte) of data received from the peripheral device and place it in itsproper position in data in register 425 so that it is properly alignedon the address/data lines for transfer to the main memory. It should befurther noted that one, or the other, or both bytes of data may bewritten into main memory. Which of the two, or both, bytes is writteninto main memory is controlled by signals PWRTB0+ and PWRTB1+ which areset by address and range logic 445 during DMA data transfers.

I/O CONTROLLER TIMING LOGIC

As discussed hereinbefore, each IOC on a system bus is allocated a 500nanosecond bus cycle slot during which it may make system bus requests.Each IOC on the bus determines when it is that IOC's bus cycle slot byusing the PTIME3- and BCYCIN- signals from the system bus. The primarytime 3 (PTIME3-) signal is distributed to each IOC on the system busesand is in the binary ZERO state for a hundred nanoseconds of the buscycle and in the binary ONE state for the 400 nanoseconds of a systembus cycle as is shown in FIG. 13. FIG. 13 also shows that the bus cyclein (BCYCIN-) signal is a binary ONE for a period of 500 nanoseconds fromthe trailing edge of one primary time 3 pulse to the trailing edge ofthe next primary time 3 pulse. It should be noted that the system buscycle in signal (BCYCIN-) of a particular IOC is the system bus cycleout (BCYCOT-) signal of the preceding IOC (i.e., the neighboring IOCwhich is closer to the CPU on the system bus).

Referring now to FIG. 39, the operation of timing logic 400 will bediscussed in detail. My time 3 flip-flop 408 and cycle in flip-flop 410are initially set (i.e., a binary ONE appears at the Q outputs thereof).My time 3 flip-flop is reset only during the primary time 3 time periodof the current IOC. Cycle in flip-flop 410 is reset only during thecycle in time period of the current IOC. It should be noted that theterms current IOC, previous IOC and next IOC refer to the relativephysical positions of the various IOCs on the system bus. Thus withreference to FIG. 1, if the current IOC is I/O controller 208 then theprevious IOC is I/O controller 206 and the next IOC is I/O controller210. The previous IOC is the adjacent IOC that is closer to the centralprocessor on the system bus. The next IOC is the adjacent IOC that isfurther away from the CPU. Referring now to FIG. 39, it can be seen thateach time the system bus primary time 3 signal (PTIME3-) transitionsfrom the binary ONE to the binary ZERO state, the output of the AND gate412 (signal PTIME3+20) will transition from a binary ZERO to the binaryONE state and clock my time 3 flip-flop 408. Initially, with bus cyclein time signal BCYCIN- being a binary ONE, indicating that it is not thecycle in time of the previous IOC, the flip-flop 408 will be set. Withmy time 3 flip-flop 408 being set or remaining set, the Q output thereof(signal DMYTM3-) will be a binary ONE which will be clocked into cyclein flip-flop 410 by signal PTIME3-30 transitioning from the binary ZEROto the binary ONE state at the end of primary time 3. Signal PTIME3-30is the output of inverter 414 which inverts the outputs of AND gate 412.The relationship of clocking signals PTIME3+20 and PTIME3-30 to theprimary time 3 system bus signal (PTIME3-) can be seen by referring toFIG. 40. FIG. 40 shows that when PTIME3- is a binary ONE, PTIME3+20 is abinary ZERO and PTIME3-30 is a binary ONE. It should be noted that theapproximate 5 to 10 nanosecond delays associated with each of the logicelements is being ignored for the purposes of this discussion.

Referring now to FIGS. 39 and 40, at time A, the beginning of the firstprimary time 3 period, the transition of PTIME3+20 from the binary ZEROto the binary ONE state will set flip-flop 408 by gating the BCYCIN-signal at the data (D) inputs thereof onto the outputs (Q and Q)thereof. If flip-flop 408 had been set previous to time A, its statewill not be changed at time A. At time B, the end of the first primarytime 3 period, the BCYCIN- signal on the system bus will transition fromthe binary ONE to the binary ZERO state indicating the start of theprevious IOC's system bus cycle in time. At time C, the beginning of thesecond primary time 3 period, the PTIME3+20 signal again gates the Dinput of flip-flop 408 onto the outputs thereof and at this time,because the BCYCIN- signal on the bus is a binary ZERO, flip-flop 408 isreset and the Q output thereof, signal DMYTM3- becomes a binary ZERO. Attime D, the end of the second primary time 3 period and the beginning ofthe current IOC's system bus cycle in time, the clocking signal(PTIME3-30) to cycle in flip-flop 410 transitions from the binary ZEROto the binary ONE state and gates the D input thereof onto the Q outputsthereof. At time D, the D input of flip-flop 410 (signal DMYTM3-) is abinary ZERO and therefore flip-flop 410 is reset indicating thebeginning of the current IOC's system bus cycle in time.

The resetting of cycle in flip-flop 410 results in the Q output therof,signal BCYCOT-, becoming a binary ZERO which when input to the set (S)input of my time 3 flip-flop 408, results in flip-flop 408 being setmaking the Q output thereof, signal DMYTM3-, a binary ONE. At time E,the end of the third primary time 3 period, the clocking signal(PTIME3-30) to flip-flop 410 again transitions from the binary ZERO tothe binary ONE state thereby clocking the binary ONE signal at the Dinput thereof onto the Q output thereof thereby settIng flip-flop 410causing signal BCYCOT- to become a binary ONE. This transition of signalBCYCOT- from the binary ZERO to the binary ONE state completes thecurrent IOC system bus cycle in time period and allows the next IOC tobegin its cycle in time.

Referring once again to FIG. 39, it can be seen that cycle in flip-flop410 will be set initially by the occurrence of the PCLEAR- signal on thebus being a binary ZERO. The PCLEAR- signal is set by the centralprocessor unit during system initialization to clear all peripherals onthe system buses. When the PCLEAR- signal is a binary ZERO, the outputof AND gate 416 is a binary ONE as is the output of AND gate 418, signalPCLEAR+20. Inverter 420 inverts the output of AND gate 418. When theoutput of AND gate 418 is a binary ONE, inverter 420 produces a binaryZERO at the set (S) input of cycle in flip-flop 410 thereby initializingthe Q output thereof to a binary ONE. This ability for the PCLEAR-signal to set cycle in flip-flop 410 is used by the system during amaster clear operation to abort any input/output operation in progressat the time of the master clear.

Once again referring to FIG. 40, it can be seen that the Q output of mytime 3 flip-flop 408 (signal DMYTM3-) is reset only during the primarytime 3 period of the current IOC (time C to D) and that the Q output ofcycle in flip-flop 410 (signal BCYCOT-) is reset only during the currentIOC's system bus cycle in time period of 500 nanoseconds (time D to E).

I/O CONTROLLER REQUEST LOGIC

Now referring to FIG. 39, the operation of the DMC request logic 402-1will be discussed in detail. For simplicity, the DMA/DMC request logic402 Of FIG. 38 is shown as DMC request logic 402-1 in FIG. 39 with thefollowing discussion of FIG. 39 assuming that the I/O controller is aDMC IOC and therefore the data transfer request logic will make a DMCrequest by use of the signal PDMCRX-. If the IOC was of the DMA typecontaining address and range logic (445 in FIG. 38), the data transferrequest logic would make a DMA request by use of the signal PDMARX-.

Initially, need DMC flip-flop 424 is reset indicating that the IOC doesnot currently need a DMC cycle to transfer a byte of data to or from aperipheral connected to the IOC. Later, when device logic 407 (FIG. 38)determines that a byte of data is needed to be read from or written intomain memory, signal DMCINC- at the D input of flip-flop 424 becomes abinary ONE. Still later, when the device logic 407 clocks the data inputsignal of flip-flop 424 to the outputs thereof by transitioning theclocking signal DERSWT- from the binary ZERO to the binary ONE stateneed DMC flip-flop 424 is set and causes the Q output thereof (signalDRQAOK+) to become a binary ONE. Once the need DMC flip-flop 424 is set,the IOC will request a DMC bus cycle during its next system bus cycle intime if another IOC on the same system bus is not already requesting aDMC cycle. Thus, if another IOC on the same system bus is not requestinga DMC cycle, the DMC request signal PDMCRX- on the system bus (denotedas signal DDMCRX- internally to the IOC) will be a binary ONE indicatingthat a DMC cycle is not being requested by an IOC on that system bus.With a binary ONE at two of the three inputs to NAND 426, i.e., DMCrequest flip-flop not set and need DMC flip-flop set, the occurrence ofa binary ONE signal at the third input, signal DMYTM3+, will cause theoutput thereof, signal DMYDMC- to become a binary ZERO thereby settingDMC request flip-flop 428. As shown in FIG. 40, signal DMYTM3- becomes abinary ZERO (therefore signal DMYTM3+ becomes a binary ONE) at time Cwhich is the beginning of primary time 3 for the current IOC's systembus cycle in time causing the system bus DMC request line signal PDMCRX-to become a binary ZERO.

Now referring to FIG. 39, it can be seen that the setting of DMC requestflip-flop 428 results in the Q output thereof, signal DMDMCF+, becominga binary ONE which in turn causes the output of NAND gate 430 to becomea binary ZERO thereby requesting a DMC cycle on system bus line PDMCRX-.Once the DMC cycle request line is set to a binary ZERO, it can be seenby reference to FIG. 39, that no other DMC IOC on that system bus willbe able to request a DMC cycle on its behalf until the DMC request lineis reset by the current IOC. It should be noted that each DMC IOC on thesystem buses has DMC request logic similar to that shown in FIG. 39.

The DMC request flip-flop 428 of the current IOC can be reset by theoccurrence of either of two events. A master clear operation will resultin the system bus clear signal PCLEAR- becoming a binary ZERO resultingin the output of AND gate 418 (PCLEAR+20) becoming a binary ONE which isone input to NOR gate 422. When signal PCLEAR+20 becomes a binary ONE,the output of NOR gate 422, signal DMCCLR- will become a binary ZEROwhich in turn will reset need DMC flip-flop 424 and DMC requestflip-flop 428. This resetting of flip-flops 424 and 428 via the clearsignal results in the clearing of any stored, but not yet acted upon,need DMC request and the resetting of the system bus DMC request line ifit is currently set by the DMC request flip-flop 428.

The second method by which flip-flops 424 and 428 can be reset is inresponse to an answer DMC command (ASDMC) being encoded on bus linesRDDT29+ through RDDT31+ which results in the setting of DMC linkflip-flop 454. When DMC link flip-flop 454 is set, the output thereof,signal DDMCCY+, becomes a binary ONE which will clock the binary ZERO atthe D input of DMC request flip-flop 428 onto the outputs thereofthereby resetting DMC request flip-flop 428. This also results in theoutput of NOR gate 422 becoming a binary ZERO which in turn causes theresetting of need DMC flip-flop 424. Thus, as will be seen hereinafter,when the CPU responds to the requesting IOC with the answer DMC commandon the system bus, the requesting IOC's DMC request flip-flop 428 isreset as is its need DMC flip-flop 424. The resetting of DMC requestflip-flop 428 results in the DMC request line (signal PDMCRX-) on theparticular system bus to which the IOC is attached becoming a binary ONEthereby allowing another IOC, or the resetting IOC, to make a DMCrequest on that particular system bus by setting its DMC requestflip-flop. If more than one IOC on a particular system bus has a needDMC flip-flop set, the first IOC to be granted its cycle in time on thesystem bus will be the IOC which is allowed to make the DMC request bysetting its DMC request flip-flop. For example, in reference to FIG. 13,if the third IOC was granted a DMC data transfer request and during theprocessing of the third IOC's DMC transfer request the fourth IOC andthe second IOC set their respective need DMC flip-flops and the DMCrequest flip-flop of the third IOC is reset during the system bus cyclein time of the first IOC, then the second IOC will be allowed to set itsDMC request flip-flop during its system bus cycle in time. The fourthIOC will have to wait until a later one of its system bus cycle in timesin which it finds that the DMC request line on its system bus has notalready been set by another IOC on the same system bus before the fourthIOC can set its DMC request flip-flop and request a DMC data transfercycle.

I/O CONTROLLER INTERRUPT REQUEST LOGIC

Again referring to FIG. 39, the operation of interrupt request logic 404will be discussed. Interrupt request logic 404 operates in an analogousmanner to that of DMC request logic 402-1. That is, need interruptflip-flop 434 and interrupt request flip-flop 438 are initially resetproducing a binary ZERO at their respective Q outputs thereof. When theI/O controller determines that an interrupt is needed, the clockingsignal DAINOK+ of need flip-flop 434 transitions from the binary ZERO tothe binary ONE state clocking the binary ONE at the data input thereofonto the Q output thereof thereby setting the flip-flop and causing thesignal DBINOK+ to become a binary ONE. Later, during the cycle in timeof the particular IOC, the occurrence of the timing signal DMYTM3+becoming a binary ONE will enable NAND gate 436 and cause the outputthereof to become a binary ZERO if the third input thereof, signalDINTRX-, is a binary ONE. Signal DINTRX- will be a binary ONE if theinterrupt request line on the particular system bus, signal PINTRX-, isa binary ONE indicating that no other IOC on that particular system bushas already set its interrupt request flip-flop. If all three inputs toNAND gate 436 are a binary ONE, the output thereof, signal DMYINT-,becomes a binary ZERO thereby setting interrupt request flip-flop 438and causing the Q output thereof, DMINTF+, to become a binary ONE.Setting of interrupt request flip-flop 438 causes the output of NANDgate 440, signal DINTRX-, to become a binary ZERO. This signal, DINTRX-,is the same as the interrupt request signal, PINTRX- on the system bus.The setting of interrupt request flip-flop 438 thereby precludes anyother IOC on the same system bus from making an interrupt request untilthe interrupt request flip-flop of the currently requesting IOC isreset. As will be seen hereinafter, when the CPU responds with an answerinterrupt command (ASINT) on system bus lines RDDT29+ through RDDT31+the interrupt link flip-flop 450 is set causing the Q output thereof,signal DINTCY+, to become a binary ONE. Signal DINTCY+ becoming a binaryONE resets interrupt request flip-flop 438 by clocking the binary ZEROat the data input thereof onto the outputs thereof resulting in theresetting of the system bus interrupt request line (signal PINTRX-).Signal DINTCY+ becoming a binary ONE in response to the answer interruptcommand from the CPU also results in the output of NOR gate 432 (signalDINSTS-) becoming a binary ZERO thereby resetting need interruptflip-flop 434. Alternatively, as discussed with respect to DMC requestlogic 402-1, the occurrence of a master clear from the CPU will causesignal PCLEAR+20 to become a binary ONE and thereby cause the output ofNOR gate 432 to become a binary ZERO which in turn will reset needflip-flop 434 and interrupt request flip-flop 438 thereby aborting anyinterrupt request currently in progress. This setting and resetting ofthe system bus interrupt request line PINTRX- is also illustrated inFIG. 23 which shows the system bus I/O interrupt sequence.

I/O CONTROLLER REQUEST RESET LOGIC

Again referring to FIG. 39, the operation of the IOC request reset logic406 will be discussed in detail. Command decoder 442 is used to decodethe system bus commands generated by the CPU under firmware control andbinary encoded on system bus lines RDDT29+ through RDDT31+. Commanddecoder 442 is a three-to-eight line decoder of the type number SN74S138 manufactured by Texas Instruments Incorporated of Dallas, Tex.and described in their publication entitled "The TTL Data Book forDesign Engineers", Second Edition. Of the three enabling (EN) inputswhich are ANDed together to enable the command decoder, only one isvariable and that one is connected to system bus line PIOCTX- such thatwhen a binary ZERO appears on that line decoder 442 is enabled and willdecode the three binary inputs (I1, I2 and I4) and produce a binary ZEROat one of the eight outputs (Q0-Q7). The binary encoding of the threecommand lines is shown in FIG. 18 which describes the eight system buscommands. For example, if a binary 011 is encoded on system bus linesRDDT29+ through RDDT31+, a binary ZERO will appear at the Q3 output ofcommand decoder 442 making the signal DASDMC- a binary ZERO. Thus, whencommand strobe signal PIOCTX- becomes a binary ZERO, one of the eightoutputs of command decoder 442 becomes a binary ZERO and the other sevenoutputs thereof remain a binary ONE. Before command strobe signalPIOCTX- becomes a binary ZERO, i.e., when the enable signal is a binaryONE, all of the outputs of command decoder 442 are a binary ONE. Thegeneration of command strobe signal PIOCTX- is controlled by CPUfirmware word bits 32 through 34 as described hereinbefore and shown inFIG. 35C and FIG. 37D. With reference to FIG. 37D it can be seen thatCPU firmware word bits 27 and 28 control which of the two or both systembus strobe command lines are enabled. That is, the microprogrammer ofthe CPU firmware has under his control through the use of bits 27 and 28which of the system buses will receive a command strobe and through theuse of bits 29 through 31 (see FIG. 37E) the command which will bestrobed to the selected system bus, either system bus A or system bus Bor both system buses A and B. Therefore, if a DMC request is being madeon system bus A and another DMC request is being made on system bus B,the CPU firmware can be microprogrammed to respond with an answer DMCcommand on system bus lines RDDT29+ through RDDT31+. The ability tocontrol the system bus command strobe to one or the other or both systembuses is important in that the system bus command on lines RDDT29+through RDDT31+ is broadcast on both system buses simultaneously and itis the bus command strobe signal (PIOCTX-) which is gated to only one ofthe two buses so that only one of the possible two requesting IOCs willbe answered. As seen in FIG. 21, the command strobe signal PIOCTX-transitions from the binary ONE to the binary ZERO state at the end ofprimary time 0 thus enabling one of the eight outputs of command decoder442.

Returning now to FIG. 39, it can be seen that one of the inputs of ANDgate 452 is signal DASDMC- from command decoder 442. The other input toAND gate 452 is signal DMDMCF- which is the Q output from DMC requestflip-flop 428. Thus, it can be seen that if an answer DMC command isencoded on system bus lines RDDT29+ through RDDT31+ and command strobeline PIOCTX- is a binary ZERO on the same system bus, the signal DASDMC-will be a binary ZERO partially enabling AND gate 452. If the DMCrequest flip-flop 428 of the IOC is set, the bottom input to AND gate452 will be enabled (a binary ZERO) thus fully enabling AND gate 452 andproducing a binary ONE at its output, (signal DDMCCS+). With a binaryONE appearing at the data (D) input of D-type flip-flop 454, theoccurrence of the clocking signal DMYLKC+ transitioning from a binaryZERO to a binary ONE state will set the DMC link flip-flop 454 andresult in signal DDMCCY+ becoming a binary ONE.

As seen hereinbefore, the transition of the signal DDMCCY+ from a binaryZERO to a binary ONE state results in the clocking of the DMC requestflip-flop 428 and the resetting thereof which results in the DMC requestline PDMCRX- on the system bus becoming a binary ONE. As also seenhereinbefore, when signal DDMCCY+ becomes a binary ONE the need DMCflip-flop 424 is also reset.

Turning now to the generation of the clocking signal DMYLKC+ whichclocks: command link flip-flop 446, interrupt link flip-flop 450, DMClink flip-flop 454 and IOC link flip-flop 468. As seen hereinbefore, theoutput of AND gate 452 will be a binary ONE if both inputs to it are abinary ZERO indicating that the IOC is receiving an answer DMC commandfrom system bus lines RDDT29+ through RDDT31+ and that the particularI/O controller's DMC request flip-flop 428 is set. If these conditionsare met, one of the inputs to OR gate 458 will be a binary ONE therebymaking the output of it (signal DMYLKS+) a binary ONE. A binary ONE atthe data input of reset request flip-flop 460 will be clocked onto theoutput at primary time 3 when the clocking signal PTIME3+20 transitionsfrom a binary ZERO to a binary ONE state. The setting of reset requestflip-flop 460 results in the output (signal DMYLKC+) transitioning froma binary ZERO to a binary ONE and results in the clocking of flip-flops446, 450, 454 and 468. At any given point in time, only one of the threeflip-flops 446, 450 and 454 will be set. The setting of one of thesethree flip-flops depends on the command encoded on the system bus linesand which of the associated request flip-flops (i.e., CPU command 470,interrupt request 438, or DMC request 428) is set.

From the above discussion it can be seen that AND gate 452 serves a dualpurpose. The first purpose is to provide a binary ONE at the data inputof DMC link flip-flop 454 if the I/O command encoded on the system busis an answer DMC command and if the DMC request flip-flop of thatparticular IOC is set. If that particular IOC's DMC request flip-flop isnot set, the setting of DMC link flip-flop 454 is not required to resetan unset (already reset) DMC request flip-flop 428. The second purposeof AND gate 452 is to generate one of the three signals which are ORedtogether by OR gate 458 the output of which is used in the setting ofreset request flip-flop 460. In turn the output of flip-flop 460 is usedto clock flip-flops 446, 450 and 454. Thus, this clocking signal DMYLKC+is generated only if an I/O controller's: DMC request flip-flop 428 isset and an answer DMC command is received, or interrupt requestflip-flop 438 is set and an answer interrupt command is received, or CPUcommand flip-flop 470 is set and an answer command is received on thesystem bus command lines RDDT29+ through RDDT31+. These later twoconditions are established by AND gate 444 and AND gate 448, the outputsof which are respectively connected to the data inputs of command linkflip-flop 446 and interrupt link flip-flop 450. Command link flip-flop446 will therefore be set if CPU command flip-flop 470 is set and ananswer command is received from the system bus. Correspondingly,interrupt link flip-flop 450 will be set if the interrupt requestflip-flop 438 is set and an answer interrupt command is received on thesystem bus. The setting of the interrupt link flip-flop 450 will resultin the signal DINTCY+ becoming a binary ONE which in turn results in theresetting of interrupt request flip-flop 438 and need interruptflip-flop 434 as discussed hereinbefore. The setting of CPU commandflip-flop 470 results in signal DMCMDF- becoming a binary ZERO which isin turn used by the IOC in developing the proceed or busy signals whichare sent to the CPU by the I/o controller to indicate whether or not theIOC is in a condition to proceed with the command dialogue from the CPU.

CPU command flip-flop 470 is set by the IOC in response to receiving aCPU command on system bus lines RDDT29+ through RDDT31+ along with theintended IOC's channel number on system bus lines BUSX00+ throughBUSX09+. The CPU command to IOC sequence shown in FIG. 20. During thetime which the CPU command is encoded on the RDDT system bus lines, theIOC compares the channel number on the bus data lines (BUSX00+ throughBUSX09+) with that of a manually settable switch in the IOC which hasbeen preset to indicate the channel number of the particular IOC. If thechannel number on the system bus equals that of the IOC, the CPU commandflip-flop 470 is set by signal DMYCMD- transitioning from a binary ZEROto a binary ONE at the clock input, clocking the binary ONE at the datainput. Setting flip-flop 470 causes the Q output signal DMCMDF- tobecome a binary ZERO partially enabling AND gate 444 which will be fullyenabled upon the occurrence of an answer command encoded on the RDDTsystem bus lines. CPU command flip-flop 470 is reset by signal DCMDR3-on the occurrence of a master clear on the system bus (signal PCLEAR+20via NOR gate 472) or the occurrence of an end-of-link command (EOFLK) onthe RDDT system bus lines at primary time 3 if command link flip-flop446 is set (i.e., the output of AND gate 474 will be a binary ONE duringthe occurrence of an end-of-link terminating the CPU command system bussequence). Note, signal PTIME3+40 is produced by inverter 476 invertingprimary time 3 signal PTIME3-30 and signal DEOFLK+ is produced byinverter 478 inverting end-of-link signal DEOFLK-.

Continuing now to discuss FIG. 39, it can be seen that IOC linkflip-flop 468 is clocked by the output of reset request flip-flop 460which also clocks flip-flops 446, 450 and 454. With a binary ONE at thedata input of IOC link flip-flop 468, IOC link flip-flop 468 will be setwhenever one of the link flip-flops 446, 450 and 454 is set. WheneverIOC link flip-flop 468 is set, the Q output of it, signal DMYLNK-, beinga binary ZERO partially enables AND gate 456. The other input of ANDgate 456 is the signal DEOFLK- which will be a binary ZERO when theend-of-link command is encoded on the system bus RDDT lines. Thus, theoutput of AND gate 456, signal DMYEND+, will be a binary ONE wheneverthe end-of-link command appears on the system bus and one of the threelink flip-flops 446, 450 or 454 is set thus assuring that the linkcommand on the system bus is directed to this particular IOC. Withsignal DMYEND+ being a binary ONE, the output of OR gate 462, signalDEOLK+, will be a binary ONE. The binary ONE at the D1 input of end linkregister 464 will be clocked to the Q1 output thereof and held there atthe primary time 3 period of the bus cycle in which the end-of-linkcommand is on the system bus. During primary time 3 of the next systembus cycle, the signal DLKD1+, being a binary ONE at the Q1 output and atthe D2 input of end link register 464, will be clocked and held at theQ2 output thereof and signal DELKD2+ will be a binary ONE. With DELKD2+being a binary ONE, the output of inverter 466, signal DELKD2- willbecome a binary ZERO, thereby resetting IOC link flip-flop 468 and theother link flip-flops 446, 450 and 454 and terminating the current linksequence in which the IOC was linked to the CPU. In addition to anend-of-link sequence resetting link flip-flops 446, 450, 454 and 468, amaster clear on the system bus will also clear these flip-flops byapplying the PCLEAR+20 to OR gate 462 which will in turn, after twoprimary time 3 periods, result in the resetting of these flip-flops.

The purpose of end-of-link register 464 is to delay the resetting of thelink flip-flops for one system bus cycle time. It should be noted thatthe output thereof after inverter 466, signal DELKD2-, remains at abinary ZERO state for one full system bus cycle time from the beginningof one primary time 3 until the end of the next primary time 2 such thatlink flip-flops 446, 450, 454 and 468 can not be set until signalDELKD2- at their reset inputs becomes a binary ONE. Therefore, linkflip-flops 446, 450, 454 and 468 cannot be set until two system buscycles have passed following the system bus cycle in which theend-of-link command was encoded on system RDDT lines. In practice, thisis not a limitation because the CPU bus commands on system bus RDDTlines are generated by CPU firmware and the CPU must perform one or morefirmware steps (i.e., system bus cycles) between the processing of theend-of-link of the previous command and the answering of any outstandingbus requests by an answer command, answer interrupt, answer DMA oranswer DMC bus command. As seen hereinbefore, the link flip-flopclocking signal DMYLKC+ is generated by setting reset request flip-flop460. In order to generate a subsequent clocking signal, which requiresthat the signal DMYLKC+ transition from the binary ZERO to the binaryONE state, flip-flop 460 must be reset. Flip-flop 460 is reset duringthe next primary time 3 of the following bus cycle by insuring that thedata input thereof, signal DMYLKS+ is a binary ZERO. This will be thecase if the bus command encoded on the system bus RDDT lines is not ananswer command, answer interrupt or answer DMC command thereby assuringthat the output of OR gate 458 is a binary ZERO. This is accomplished byinsuring that the CPU firmware microprogram does not encode twoconsecutive microinstructions which generate answer commands on thesystem bus. In practice, this is no restriction in light of the bussequence dialogs.

DMA IOC REQUEST AND RESET LOGIC

A given I/O controller within the system is either a DMA or a DMCcontroller. The above discussion with respect to the request logic andrequest reset logic and link logic has been in terms of a DMC I/Ocontroller but the logic is equally applicable to that of a DMA I/Ocontroller. That is, in FIG. 39, need flip-flop 424, request flip-flop428, and link flip-flop 454 could equally have been DMA flip-flops inwhich case the answer DMA signal (DASDMA-) from command decoder 442would have been used to set the link flip-flop 454 via AND gate 452.

I/O CONTROLLER SYSTEM BUS REQUEST AND LINK LOGIC SUMMARY

Review briefly now the operation of the logic shown in FIG. 39. Timinglogic 400 is mainly concerned with generating the timing signals usedwithin a particular IOC and passing on the cycle in timing signal BCYCINfrom the previous IOC on the bus and delaying it for 500 nanosecondsbefore passing it on as signal BCYCOT onto the next IOC on the systembus. In addition, the timing logic 400 generates the primary time 3signals PTIME3+20 which is a binary ONE during every system bus cycleand signal DMYTM3+ which is a binary ONE only during the primary time 3period of a particular IOC's system bus cycle in time. DMA request logic(now shown), DMA/DMC request logic 402 (FIG. 38), and interrupt requestlogic 404 are used to request a DMA, DMC, or interrupt request of theCPU respectively. The setting of an IOC's request flip-flop, forexample, DMC request flip-flop 428, is allowed only during thatparticular IOC's cycle in time on the system bus thereby eliminating thepossibility of more than one IOC on that particular system bus (A or B)trying to make a particular type request. Because the DMA, DMC andinterrupt request lines on system bus A are separate and distinct fromthose on system bus B, an IOC on system bus A can, for example, make aDMC request at the same instant in time that an IOC on system bus B ismaking a DMC request because each IOC is currently experiencing itscycle in time. It should be further noted that a given IOC on aparticular system bus may concurrently make an interrupt request and aDMC request (or a DMA request) because nothing in the request logicprecludes more than one request logic within a given IOC being set at agiven instance in time. It should be noted that the priorities betweendifferent request types (i.e., DMA, DMC or interrupt) are sorted out byCPU as well as competing requests of the same type between system bus Aand B. Further examination of the request logic will show that during aparticular cycle in time period a first IOC may reset its DMC requestflip-flop 428 and a second IOC on the same system bus may set its DMCrequest flip-flop 428 if it is the second IOC's cycle in time period andif its need DMC flip-flop 424 is set. This can occur even though bothIOCs on the same system bus will be receiving the answer DMC command onsystem bus RDDT lines. Only the first IOC will generate the DMC requestflip-flop reset signal DDMCCY+ from DMC link flip-flop 454 because onlythe first IOC's DMC request flip-flop 428 was set at the time theclocking signal PTIME3+20 clocks reset request flip-flop 460. At theleading edge of PTIME3+20 in the second IOC on the same system bus, theoutput of AND gate 452, signal DDMCCS+ will be a binary ZERO andconsequently the output of OR gate 458, signal DMYLKS+ will be a binaryZERO so that reset request flip-flop 460 will not be set by clockingsignal PTIME3+20. Because the second IOC's reset request flip-flop 460will not be set, the Q output thereof, signal DMYLKC+, will remain abinary ZERO and DMC link flip-flop 454 will not be clocked and willremain reset.

The second IOC's output of OR gate 458, signal DMYLKS+, will be a binaryZERO because the other two inputs there to, signals DINTCS+ and DCMDCS+,will also be a binary ZERO at the leading edge of PTIME3+20 because atany given instant in time the CPU can only be giving one type of commandon the system bus RDDT lines. Therefore only one output of commanddecoder 442 will be a binary ZERO and consequently only one, if any,output of AND gate 444, 448 and 452 can be a binary ONE at PTIME3+20.That is, reset request flip-flop 460 will be set and generate linkflip-flop (446, 450, 454 and 468) clocking signal DMYLKC+ only if arequest flip-flop (470, 438 or 428) is set and the corresponding command(ASCMD, ASINT or ASDMC respectively) is received from the CPU on thesystem bus RDDT lines.

Although a given peripheral device connected to a particular IOC willnot normally, in any instant in time, be making both a DMA (or DMC)request in conjunction with an interrupt request, it is possible to havemultiple peripheral devices connected to one IOC in which case thatparticular IOC could be requesting both an interrupt and a data transfer(DMA or DMC) at a particular instance in time, in which case it wouldhave more than one of its request flip-flops set.

Once an IOC makes a system bus request, the CPU firmware will respond byan answer command on system bus RDDT lines. As shown in request resetlogic 406, the response of the CPU to the system bus request producestwo actions. The first action is that the request flip-flop of therequesting IOC is reset and the corresponding link flip-flop is set.Thus continuing with the DMC example, if an IOC on system bus A makes aDMC request the CPU firmware will answer with an answer DMC command onsystem bus RDDT lines for both system buses A and B, but will onlyrespond on system bus A with the controller lines PIOCTX- (PIOCTA- inthis case) being set to a binary ZERO. Thus, although the answer DMCcommand is broadcast both on system bus A and system bus B only the IOCson system bus A will have their command decoders 442 enabled by systembus line PIOCTA- and so that only the IOCs on system bus A couldpossibly respond to the answer DMC command. Because only one IOC onsystem bus A can have its DMC request flip-flop set, only that IOC onsystem bus A will set its DMC link flip-flop 454 and reset its DMCrequest flip-flop 428. It should be noted that the broadcasting of theanswer DMC command on the system bus RDDT lines and the resetting ofthat particular IOC on system bus A's DMC link flip-flop 454 can occurduring any primary time 3 and need not necessarily occur during the DMCrequesting IOC's cycle in time. Once a particular IOC's link flip-flopis set, that particular IOC and CPU are linked together and all furtherdialog on system bus address/data lines (BUSX00+ through BUSX15+) isdedicated to the particular request being processed. Thus, on systembuses A and B only one IOC will have one of its link flip-flops 446, 450or 454 set and it will remain set until the end-of-link command is sentby the CPU and received by that particular IOC. Because only one IOC inthe system has a link flip-flop set, the end-of-link command can bebroadcast by the CPU firmware on both system buses and only one IOC willreset by resetting its link flip-flop.

Again, it is to be noted that the CPU may broadcast the end-of-linkcommand at any time and need not await the cycle in time period of theparticular IOC that is currently linked to the CPU. Thus, it can be seenthat the purpose of the cycle in time is to eliminate contentionproblems between IOCs on a particular system bus and that, onceinitiated, the bus dialog between IOC and the CPU can take placeirrespective of the cycle in time of the involved IOC. As discussedhereinbefore and as illustrated in FIGS. 16, 17 and 18, the normalsequence for DMC, DMA and interrupt requests is: for the IOC to initiatea bus request during its cycle in time; for the CPU to respond with ananswer at any time; followed by an end-of-link command from the CPU atany time. Further, as discussed hereinbefore and illustrated in FIG. 16,the CPU may initiate IOC activity by placing a CPU (CPCMD) command onsystem bus lines RDDT at any time and placing the particular IOC'schannel number on the system bus address/data lines. As used herein, theterm "any time" means that period of time that the system bus is notalready in use by another IOC being linked to the CPU (i.e., that timeduring which no link flip-flop of an IOC is set) without regard to thecycle in time of the involved IOC. It should be further noted that thenormal bus dialog sequence of: bus request by the IOC; followed by ananswer command from the CPU; followed by an end-of-link command from theCPU can be aborted by any point by a master clear (PCLEAR signal) whichwill reset the corresponding flip-flops and abort the sequence such thata new sequence may be initiated.

CPU LOGIC DETAILS

Having described the operation of the I/O controller logic, the CPUlogic will now be described in detail. FIG. 42 illustrates the controlstore logic described above in conjunction with the control store blockdiagram, FIG. 9, and FIG. 43 shows the CPU logic described above inconjunction with the CPU block diagram, FIG. 8.

CONTROL STORE LOGIC DETAILS

Referring now to FIG. 42 the operation of the control store logic willbe discussed in detail. When the system is initialized address register1, 246-1, and address register 2, 246-2, are reset causing the 10-bitaddress output thereby to be set to zero. With the 10-bit ROS address(RADR00+ through RADR09+) set to zero, location zero is read out of ROS1, 238-8, and ROS 2, 238-2, and loaded into local register 242 at thebeginning of primary time 0. The 48-bit microinstruction firmware wordcontained in local register 242 is then fed throughout the CPU. Two bitsin the firmware word read from ROS control the selection of one of fourinputs to address generator 1 multiplexer, 248-1, some of the outputs ofwhich are fed back to address register 1, 246-1, and the other outputsof which are fed to address generator 2, 248-2. The outputs of addressgenerator 2, 248-2 are in turn fed to address register 2, 246-2. Withthe outputs of address registers 1 and 2, 246-1 and 246-2, beingdetermined by the previous microinstruction firmware word, the addressof the next microinstruction is presented to ROS 1 and 2, 238-1 and238-2, and the next microinstruction is read and then clocked into localregister 242 at the beginning of primary time 0. In this manner, withoutthe occurrence of a hardware interrupt as described below, the currentmicroinstruction determines the next microinstruction to be read fromROS.

ROS Address Generation Logic

Now returning to address register 1 and 2, 246-1 and 246-2, the aboveoperation will be discussed in more detail. When the system isinitialized, signal PCLEAR- becomes a binary ONE which in turn, as willbe seen below, will cause PCPCLR+ to become a binary ONE and signalPCPCLR- to become a binary ZERO at the beginning of primary time 4. Withsignal PCPCLR+ being a binary ONE at one input of NOR gate 595 theoutput thereof, signal RARCLR- becomes a binary ZERO which in turnclears address register 1, 246-1, causing the 6 most significant bits ofthe 10-bit ROS address, signals RADR00+ through RADR05+, to becomebinary ZEROs. With signal PCPCLR- being a binary ZERO at the reset (R)input of address register 2, 246-2, the least significant bits of the10-bit ROS address, signals RADR06+ through RADR09+ become a binaryZEROs thereby making the 10-bit ROS address all zeros. With the enable(EN) read inputs, signal KENROS-, being a binary ZERO, ROS 1, 238-1,addresses the location specified by the 10-bit address and places the40-bit output thereof on lines RROS00+ through RROS23+ and RROS32+through RROS47+. With the enable (EN) read inputs, signals KENROS- andPENBBT+ being binary ZEROs, the ROS 2, 238-2, addresses the locationspecified by the 10-bit address and puts the 8-bit output thereof onoutput lines RROS24+FM through RROS31+FM. It should be noted that ROS 1,238-1, is composed of 10 PROMS of 4-bits by 1024 locations and that ROS2, 238-2, and boot PROM, 240, are each composed of 2 PROMs of 4-bits by1024 locations each. These 4-bit by 1024 location PROMs are of the type82S137 manufactured by Signetics Corporation of Sunnyvale, California,and described in their publication "Signetics Bipolar and MOS MemoryData Manual", incorporated herein by reference. The 48-bitmicroinstruction firmware word read from ROS 1 and 2 is clocked intolocal register 242 at the beginning of primary time 0 by signal PTIME0+.The output of local register 242, signals RDDT00+ through RDDT47+, areused throughout the CPU to control the central processor unit and arealso inverted by inverters (not shown) to produce control signalsRDDT00- through RDDT47- when required.

Control store address generator 248 of FIG. 9 is composed of addressgenerator 1 multiplexer, 248-1, and address generator 2 multiplexer,248-2, in FIG. 42. Address generator 1 multiplexer, 248-1, which iscomposed of ten 4-to-1 multiplexers determines the address of the nextmicroinstruction to be fetched from ROS 238 depending upon the branchtypes specified in bits 36 and 37 of the current firmware word. FIG. 35Dshows that if both select (SEL) inputs RDDT37+ and RDDT36+ are a binaryZERO, the branch type specified in the current microinstruction is anunconditional branch and the 10-bit ROS address of the nextmicroinstruction will be specified by the I0 inputs, signals RDDT38+through RDDT47+. If bits 36 and 37 of the current microinstructionspecify a binary 01, then a 2-way test branch is specified and the I1inputs will be selected using: four bits from address register 1, 246-1,signals RADR00+ through RADR03+; five bits from the currentmicroinstruction, signals RDDT42+ through RDDT46+; and the one bit fromthe branch on test network 254 (see FIG. 9), signal RASBT9+. If bits 36and 37 of the current microinstruction specify a binary 10, addressgenerator 1 multiplexer, selects the multiple test branch inputs, I2,and uses: 6-bits from the current microinstruction, signals RDDT38+,RDDT39+ and RDDT44+ through RDDT47+; and 4 bits from major branchnetwork 256 (see FIG. 9), signals RAMBT2+ through RAMBT5+. If bits 36and 37 of the current microinstruction specify a hardware interruptreturn branch then the I3 inputs are used and the 10-bit address isdetermined by signals RITR00+ through RITR09+ from hardware interruptreturn register 252. The outputs of address generator 1 multiplexer,248-1, signals RADM00+ through RADM09+ are valid as long as outputcontrol (F) input signal KENRAM- remains a binary ZERO.

Address generator 2 multiplexer, 248-2, is used to select whether thefour least significant bits of the 10-bit ROS address should come fromthe output of address generator 1 multiplexer, 248-1, or from hardwareinterrupt encoder 250-2. If no hardware interrupts are pending or if oneor more interrupts are pending but hardware interrupts are inhibited,signal PHINTL+ will be a binary ZERO and the I0 inputs, signals RADM06+through RADM09+ will be selected. If one or more hardware interrupts arepending and hardware interrupts are enabled, signal PHINTL+ will be abinary ONE and the I1 inputs, a binary ONE and signals PHINT7+ throughPHINT9+ will be selected and the four least significant bits of the10-bit ROS address will be determined by the encoding of the pendinghardware interrupt.

The outputs of address generator 2 multiplexer, 248-2, signals RADN06+through RADN09+, will be clocked into address register 2, 246-2, at thebeginning of primary time 2 when signal PTIME2+ transitions from thebinary ZERO to the binary ONE state. Similarly, address register 1,246-1, will contain the sixth most significant bits of the 10-bit ROSaddress as determined by the output of address generator 1 multiplexer,248-1. The 10-bit ROS address clocked into address register 1 and 2 atthe beginning of primary time 2 is then used to address ROS 1 and ROS 2to read a 48-bit firmware word which is then clocked into local register242 at the beginning of primary time 0. In this manner, the currentmicroinstruction determines the address of the next microinstruction tobe read upon the occurrence of a hardware or software interrupt.

Hardware Interrupt Logic

Continuing to refer to FIG. 42, the handling of hardware and softwareinterrupts will be discussed in detail. Once during each CPU cycle atthe end of primary time 4 interrupt request register 250-1 is clocked bysignal PTIME4-. The data inputs of interrupt request register 250-1 areconnected respectively to: interrupt request system bus B (signalPINTR2-), interrupt request system bus A (signal PINTR1-), DMC datarequest system bus B (signal PDMCR2-), DMC data request system bus A(signal PDMCR1-), DMA data request system bus A (signal PDMAR1-), DMAdata request system bus B (signal PDMAR2-), main memory refresh timeout(signal PHMRFL-), and main memory read cycle (signal PMRCYC-). The DMAand DMC data request and main memory refresh timeout output signals ofinterrupt request register 250-1 are connected to the inputs of hardwareinterrupt encoder 250-2. The output of parity error flip-flop 586,signal PHMPER-, and nonexistent memory flip-flop 592, signal PHNDXM-,are also connected as inputs to hardware interrupt encoder 250-2.Hardware interrupt encoder 250-2 is an 8-line to 3-line priority encoderof the type SN74148 manufactured by Texas Instruments Incorporated ofDallas, Texas. Hardware interrupt encoder 250-2 encodes the 8 inputlines onto the three output lines, signals PHINT7+ through PHINT9+,whenever the enable (EN) output signal is a binary ZERO. The I output,signal PHINTP+, of hardware encoder 250-2 will be a binary ONE wheneverone of the eight inputs thereof is a binary ZERO, indicating that thereis a hardware interrupt pending.

If there is a hardware interrupt pending, the enable (I) output ofhardware interrupt encoder 250-2, signal PHINTP+, will be a binary ONEpartially enabling AND gate 591. If bit 35 of the current firmware wordis a binary ONE, enabling hardware interrupts (see FIG. 35C), signalRDDT35+ will be a binary ONE fully enabling AND gate 591 and make theoutput signal PHINTL+ a binary ONE. With signal PHINTL+ being a binaryONE partially enabling AND gate 593, at the beginning of primary time 2signal PTIME2+ becomes a binary ONE fully enabling AND gate 593 andmaking the output thereof, signal PHINTC+, a binary ONE. With one inputof NOR gate 595 being a binary ONE, the output thereof, signal RARCLR-will be a binary ZERO resetting address register 1, 246-1, therebyclearing to zero the six most significant bits of the 10-bit ROSaddress. With the output of AND gate 591, signal PHINTL+ being a binaryONE, address generator 2 multiplexer 248-2, will select the I1 inputsand encode on the three least significant bits of the 10-bit ROS addressthe address corresponding to the input number of the highest priorityinterrupt pending at an input of hardware interrupt encoder 250-2. Withthe fourth least significant bit of the 10-bit ROS address being setequal to a binary ONE by a binary ONE appearing at bit zero of input I1of address generator 2 multiplexer 248-2, it can be seen that if ahardware interrupt is pending and hardware interrupts are enabled thecombined output of address generator register 1 and address generatorregister 2 will be a 10-bit ROS address within the range of 8 through15.

Thus it can be seen that if a hardware interrupt is pending and hardwareinterrupts are enabled, the outputs of hardware interrupt encoder 250-2will clear the 6 most significant bits of the 10-bit ROS address byclearing address register 1, 246-1, and by selecting the encodedpriority of the hardware interrupt via address generator 2 multiplexer248-2 and address register 2, 246-2. By use of this mechanism a 10-bitROS address for the microroutine coded to handle the particular pendinghardware interrupt is generated and the microroutine is executed.

It is the function of hardware interrupt return register 252 to save the10-bit ROS address of the next microinstruction prior to invoking ahardware interrupt microroutine. Thus, whenever a hardware interruptmicroroutine is invoked, the output of AND gate 593, signal PHINTC+ willtransition from a binary ZERO to a binary ONE state and clock hardwareinterrupt return register 252 thereby saving the 10-bit ROS address ofwhat would have been the next microinstruction to be used from ROS.Since signal PHINTC+ transitions from the binary ZERO to the binary ONEstate only once per hardware interrupt, and then only at the beginningof the hardware interrupt microroutine, hardware interrupt returnregister 252 will retain the contents of the 10-bit ROS address of themicroinstruction that would have been read from ROS if there had notbeen a hardware interrupt all during the processing of the microroutineassociated with the hardware interrupt. It should be noted that hardwareinterrupts are inhibited during the processing of the hardware interruptmicroroutine (i.e., hardware interrupts are not nested). The lastinstruction of the microroutine for processing a hardware interruptcodes a hardware interrupt return branch in bits 36 and 37 of thefirmware word causing signals RDDT36+ and RDDT37+ to be binary ONEsthereby selecting the I3 inputs of address generator 1 multiplexer,248-1, thereby causing the 10-bit ROS address of the nextmicroinstruction to be taken from the hardware interrupt return register252 which effectively returns control to the microinstruction firmwareword addressed by the 10-bit address produced by the address generator 1multiplexer, 248-1, just prior to the occurrence of the hardwareinterrupt as discussed above with respect to FIG. 34.

An examination of FIG. 42 shows that the address of the nextmicroinstruction is developed during the execution of the currentmicroinstruction. Thus, if at the beginning (before primary time 2) ofthe current microinstruction a hardware interrupt signal at one of theinputs of hardware interrupt encoder 250-2 is a binary 0, and ifhardware interrupts are enabled in the current microinstruction (bit 35is a binary ONE, signal RDDT35+), the next microinstruction fetched willbe the first microinstruction of the microroutine for servicing thehighest priority interrupt pending. If interrupts are inhibited (bit 35is a binary ZERO) by the current microinstruction, a hardware interruptwill not occur at the end of the current microinstruction and the nextmicroinstruction will be the microinstruction addressed by the ROSaddressing field of the current microinstruction.

Because microinstruction bit 35 controls what happens at the end of thecurrent microinstruction, hardware interrupt service microroutines aremicroprogrammed such that hardware interrupts are enabled during thelast microinstruction of the microroutine and the ROS addressing field(see FIG. 35D) specifies that the address of the next microinstructionis to be taken from the hardware interrupt return register 252 whichwill return control to the interrupted original microprogram (if nohardware interrupt is pending). If a hardware interrupt is pendingduring the last microinstruction of a first hardware interrupt servicemicroroutine, the address generator 1 multiplexer 248-1 will select theI3 input (i.e., the address stored in hardware interrupt return register252) and make its Q output (signals RADM00+ through RADM09+) equal to I3input. The output of address generator 1 multiplexer 248-1 will then berestored in hardware interrupt return register 252 at primary time 2when signal PHINTC+ transitions from a binary ZERO to a binary ONE.Further, address generator 2 multiplexer 248-2 will select the secondhardware interrupt address at its I1 inputs and, at primary time 2,address register 1, 246-1, will be cleared by signal RARCLR- and addressregister 2, 246-2, will be set to the second hardware interrupt address.Thus, at primary time 2 of the current microinstruction, which is thelast microinstruction of a first hardware interrupt servicemicroroutine, the address contained in address register 1 and 2, 246-1and 246-2, will be the address of the first microinstruction of a secondhardware interrupt service microroutine.

A further examination of FIG. 42 will reveal that at the end of thesecond hardware interrupt service microroutine, if there are no furtherhardware interrupts pending, control will be returned to themicroinstruction whose address is stored in hardware interrupt returnregister 252 thus returning control to the original microprogram at thepoint that it was interrupted by the first hardware interrupt. Thus itcan be seen that back-to-back (i.e., consecutive) hardware interruptscan be serviced without returning to the interrupted originalmicroprogram between the first and second hardware interrupt servicemicroroutines.

Having described how a DMA data request or DMC data request on systembus A or B will cause a hardware interrupt to ROS locations 12 through15 and invoke the associated hardware interrupt microroutine, the otherhardware interrupt logic of FIG. 42 will now be described.

Main Memory Refresh Timeout Logic

Flip-flops 572 and 574 are used to insure that a main memory refreshsignal PHMRFL- is generated every 8 to 15 microseconds. As describedabove with respect to FIG. 14, clocking signal BCNTL8+ will transitionfrom the binary ZERO to a binary ONE state each four microsecondsthereby clocking flip-flops 572 and 574. With 8 microsecond refreshflip-flop 572 being clocked each four microseconds, the output thereof,signal PMRFPM+ will be a binary ZERO for four microseconds following bya binary ONE for the next four microseconds thereby completing one cyclein eight microseconds. By cascading the output of eight microsecondrefresh flip-flop 572, signal PMRFTM+, to the data (D) input of 16microsecond refresh flip-flop 574, the output of flip-flop 574, signalPHMRFL- will be a binary ZERO for eight microseconds following by abinary ONE for eight microseconds completing one cycle in 16microseconds. When the output of 16 microsecond refresh flip-flop 474 isa binary ZERO, output signal PHMRFH- of interrupt request register 250-1will be a binary ZERO and cause a binary 101 to be encoded on hardwareinterrupt encoder 250-2 output lines PHINT7+ through PHINT9+ which inturn, when hardware interrupts are enabled, causes address generator 2multiplier 248-2 to branch to ROS location 10 which begins the mainmemory refresh time out microroutine.

Returning now to flip-flops 572 and 574 it can be seen that bothflip-flops are reset when the output of NOR gate 580, signal PMFRSH- isa binary ZERO. Signal PMRFSH- from subcommand decoder 244-2 (see FIG.43) is set to a binary ONE if bits 32 through 34 of the firmware wordare set equal to a binary 001 causing signals RDDT32+ and RDDT33+ to bebinary ZEROs and RDDT34+ to be a binary ONE. Signal PMRFSH- is invertedby inverter 576 causing signal PMRFSH+ to be a binary ONE whenever theinput is a binary ZERO. When signal PMRFSH+ is a binary ONE at the inputof NOR gate 580, the output thereof, signal PMFRSH-, will be a binaryZERO setting 8 microsecond flip-flop 572 and 16 microsecond refreshflip-flop 574 via their reset (R) inputs. Main memory refresh signalPMFRSH- is also transferred to the main memory via system bus B.Alternatively, signal PMFRSH- will be a binary ZERO whenever signalKEFRSH- is a binary ZERO and inverted via inverter 578 causing signalKEFRSH+ to be a binary ONE at the second input of NOR gate 580.

By examining the interaction of the flip-flops 572 and 574 with thehardware interrupt logic, it can be seen that the output of 16microsecond refresh flip-flop 574, signal PHMRFL-, will become a binaryZERO 8 microseconds after a memory refresh signal (PMFRSH-) has beensent to the main memory via system bus B. Refresh interrupt signalPHMRFL- will remain a binary ZERO for 8 microseconds or until it iscleared by a subsequent memory refresh signal (PMFRSH- becoming a binaryZERO). Signal PHMRFL- being a binary ZERO will cause signal PHMRFH- tobe a binary ZERO and, if hardware interrupts are permitted, cause thefirmware to branch to the microroutine which in turn will send a memoryrefresh signal (PMFRSH-) to the main memory via system bus B. Becausethe output of the 16 microsecond flip-flop 574, signal PHMRFL-, willbecome a binary ZERO 8 microseconds after the previous memory refresh,it is important that the CPU firmware be coded such that hardwareinterrupts are not inhibited for a period of longer than 7 microsecondsin order to maintain the proper MOS memory refresh rate of 15microseconds and not lose main memory information.

If the CPU firmware is not inhibiting hardware interrupts, the firstmemory refresh will occur at the end of 8 microseconds causing the CPUfirmware to branch to the main memory refresh microroutine which in turnwill cause a refresh command to be sent to the main memory on system busB via signal PMFRSH- which in turn will reset the refresh flip-flops 572and 574 and cause a second main memory refresh interrupt to be sentafter 8 more microseconds. Because main memory refresh commands may alsobe encoded within the processing of software instructions, whenever theCPU microprogrammer determines that the main memory will not be used forthe next two CPU cycles, not all refresh signals are generated by theexpiration of the main memory refresh time (i.e., flip-flops 572 and574). The purpose of refresh flip-flops 572 and 574 is to insure that ata minimum a refresh signal is sent to the main memory once every 15microseconds but in actual practice the CPU firmware is coded such thatduring the processing of software instructions if it is known that themain memory will not be used during the next two CPU cycles, refreshcommands are encoded within the CPU firmware microinstructions togenerate main memory refresh signals more frequently than once each 15microseconds.

Main Memory Parity Error Logic

During each main memory read operation, the memory generates two paritybits per 16-bit word (1 parity bit per 8-bit byte) read from memory andcompares the generated parity with that read from memory. If thegenerated parity bits do not agree with the parity bits read frommemory, the main memory generates a parity error signal which is sentvia system bus B to the CPU on line MEMPER-.

Now continuing with FIG. 42, the occurrence of a parity error detectedby the main memory will result in the setting of parity error flip-flop586, which in turn may result in a hardware interrupt. If the mainmemory detects a parity error upon the memory read operation, mainmemory will set signal MEMPER- to a binary ZERO on system bus B. Withsignal MEMPER- being a binary ZERO the output of inverter 596 signalPMMPAR+ will be a binary ONE and signal PMMPAR+ is inverted by inverter598 and sent to system bus A as signal PMMPAR-. If the memory read wasperformed in response to a DMA data request, these parity error signals,signal PMMPAR- on system bus A and signal MEMPER- on system bus B willbe strobed into the status register of the requesting DMA IOC so thatthe error may be reported to the software program when the status of theI/O transfer is requested.

The main memory parity error signal PMMPAR+ at the data (D) input ofparity error flip-flop 586 will be clocked when clocking signal PARFMD-transitions from a binary ZERO to a binary ONE. The clocking of thebinary ONE at the data input of parity error flip-flop 586 will resultin output signal PHMPER- becoming a binary ZERO and, when hardwareinterrupts are enabled, cause a hardware interrupt. Clocking signalPARFMD-, which is the output of NAND gate 582, will transition from thebinary ZERO to the binary ONE state at the beginning of primary time 4when signal PTIME4+ becomes a binary ONE if signal PBSFMD- is a binaryZERO. Signal PBSFMD- is a binary ZERO during the second cycle of a twocycle main memory read operation as can be seen in FIG. 19. SignalPBSFMD- is used by the main memory to enable the data read from mainmemory onto system bus B.

Parity error flip-flop 586 will remain set, thereby requesting ahardware interrupt, until it is cleared by a CPU initialize signal whichwill result in signal PCPCLR- becoming a binary ZERO at one input of NORgate 584 causing the output thereof, signal PARCLR-, to become a binaryZERO thereby resetting flip-flop 586. Alternatively, parity errorflip-flop 586 will be reset by a subsequent main memory read operation.

Nonexistent Memory Detection Logic

As mentioned above, the CPU within the system monitors each main memoryaddress as it is presented to main memory to determine whether theaddressed location is physically present in one of the main memoryboards connected to system bus B. Continuing to refer to FIG. 42, beforethe system is put into operation the address of the last location(highest address) physically present in main memory is set in memorysize switch 588 by binary encoding the five most significant bits of the16-bit address. Memory comparator 590 is of the type SN74S85,manufactured by Texas Instruments Incorporated of Dallas, Texas andcompares the five most significant bits of the address of the lastmemory location with the five most significant bits of the address fromthe internal bus 260 (signals PBUS00+ through PBUS04+) and sets outputsignal PBUSLG+ to a binary ONE if the address on the bus exceeds theaddress of the last location physically present within the system. Thiscomparison of the contents of the internal bus 260 with the address ofthe last location physically present in memory is made continuously butthe output of this comparison, signal PBUSLG+ is only clocked intononexistent memory flip-flip 592 by the transition of clocking signalPMEMGO+ from a binary ZERO to a binary ONE state.

Clocking signal PMEMGO+ transitions from a binary ZERO to a binary ONEstate at the beginning of primary time 3 (see FIG. 19) when firmwareword bit 23 is a binary ONE (see FIG. 35C). When PMEMGO+ is a binary ONEthe contents of internal bus 260 will be the address of a location inmain memory and therefore the data (D) input of nonexistent memoryflip-flop 592 will reflect whether or not the addressed location isphysically present within main memory. If the addressed location is notphysically present within main memory, signal PBUSLG+ will be a binaryONE and be clocked into nonexistent memory flip-flop 592 making the Qoutput thereof, signal PHNDXM+ a binary ONE and making the Q outputthereof, signal PHNDXM- a binary ZERO. The setting of signal PHNDXM+ toa binary ONE will cause the output of NAND gate 594, signal MEMPER-, tobecome a binary ZERO as well as signal PMMPAR- via inverters 596 and598.

Therefore, it can be seen that the detection of an attempt to address anonexistent memory location will result in memory parity error signalsMEMPER- and signal PMMPAR- becoming a binary ZERO and thereby signal I/Ocontrollers on system buses A and B that a nonexistent memory locationhas been addressed. I/O controllers making a memory access will save thestatus of these error lines in their status register for later reportingof it to the software.

The setting of nonexistent memory flip-flop 592 causes signal PHNDXM- tobecome a binary ZERO at the highest priority input of hardware interruptencoder 250-2 which in turn, when interrupts are permitted, will cause anonexistent memory hardware interrupt to occur. The processing of thenonexistent memory hardware interrupt will cause the CPU firmware tobranch to ROS location 8 and process the hardware interrupt. Thenonexistent memory microroutine performs a memory refresh operation,using main memory address zero to insure there will not be a subsequentnonexistent memory error, which in turn will cause signal PBUSLG+ tobecome a binary ZERO thereby resetting nonexistent memory flip-flop 592and clearing the pending nonexistent memory hardware interrupt.

Before leaving the nonexistent memory detection logic, it should benoted that the detection of an attempt to access a nonexistent memorylocation will result in the setting of nonexistent memory flip-flop 592and also parity error flip-flop 586 via signal PMMPAR+, the output ofinverter 596. Although the setting of nonexistent memory flip-flop 592will also cause the setting of parity error flip-flop 586, only onehardware interrupt will be processed and that will be the nonexistentmemory hardware interrupt which is of higher priority than memory parityerror interrupt, which is the next highest priority. The hardwareinterrupt priority can be seen by examining inputs to hardware interruptencoder 250-2. The memory parity error hardware interrupt will not occurbecause the main memory refresh operation performed in the nonexistentmemory hardware interrupt microroutine will clear both the non-existentmemory flip-flop 592 and parity error flip-flop 586. Therefore uponreturn from the nonexistent memory hardware interrupt microroutine,signal PHMPER- will be a binary ONE and no memory parity hardwareinterrupt will be pending.

If a nonexistent memory address or memory parity error is detectedduring a DMA or DMC data transfer, signal PTIOGO- will be set to abinary ZERO during the CPU cycle (the second cycle) following the cyclein which the memory go signal is sent to the main memory (see FIG. 19).The setting of signal PTIOGO- to a binary ZERO will cause the output ofNOR gate 584, signal PARCLR-, to become a binary ZERO resulting in theresetting of nonexistent memory flip-flop 592 and parity error flip-flop586 before hardware interrupts are enabled by the CPU firmware thuspreventing the associated hardware interrupts. This resetting offlip-flops 592 and 586 masks potential hardware interrupts associatedwith data transfers to/from I/O controllers from interrupting the CPUfirmware flow. It also assures that the occurrence of either thenonexistent memory address or memory parity error hardware interrupt isdue to a memory access conducted by the CPU firmware during the courseof executing a software instruction and not due to a DMA or DMC datatransfer, which is also conducted by the CPU firmware

Although the memory error lines PMMPAR- and MEMPER- are set when the CPUdetects any attempt to access a nonexistent memory location, in the caseof a DMC data transfer which writes data into memory, the setting of thememory error line will occur only after the DMC I/O controllers havestopped looking at the error line. By referring to FIG. 21A, it can beseen the memory error line will be set, if an error occurs, only afterthe end-of-link (EOFLK) command has terminated the CPU and IOC link.IOCs only monitor the error line when the IOC is linked to the CPU, toinsure that any error signal is directed to that particular IOC, andtherefore a non-existent memory error would go unseen by a DMC IOCduring an input data transfer cycle.

To insure that nonexistent memory errors do not go undetected, the upperbound (highest address) of the block to be transferred is checked duringthe execution of the IOLD software instruction before the DMC datatransfer is begun (see FIG. 29 and the discussion thereof). It should benoted that this special check need only be performed for DMC input(write to memory) operations. It should be further noted that duringmemory write operations the memory error line is only set fornonexistent memory errors, there being no parity check performed whenmemory is written into. For DMC output and DMA input and outputoperations, the memory error line will be set before the end-of-link,while the IOC is still monitoring the memory error line, so that nospecial check is needed in these cases.

Software Interrupt Logic

Processing of software interrupts was described above with respect toFIG. 34. Now returning to FIG. 42 the software interrupt logic will bediscussed in detail. Software interrupt encoder 257-1 is of the typeSN74148 manufactured by Texas Instruments Incorporated of Dallas, Texas.Going from the highest to the lowest priority software interrupt, theinputs of software interrupt encoder 257-1 will be a binary ZEROdepending upon the status of the associated software interruptcondition. Signal PFITRP- will be a binary ZERO if a register overflowis detected and the corresponding bit is set in mask (M1) register (seeFIG. 4). Signal PFAILF- will be a binary ZERO if an impending powerfailure is detected. Signal PFINT1- will be a binary ZERO if an I/Ocontroller on system bus A is making an interrupt request and signalPFINT2- will be a binary ZERO if an I/O controller on system bus B ismaking an interrupt request. Signal PFITKL- will be a binary ZERO if atimer has expired.

The inputs of the software interrupt encoder 257-1 are binary encodedonto the three outputs thereof, signals PFINT3+ through PFINT5+. Theenable (I) output signal PFINTP- of software interrupt encoder 257-1will be a binary ZERO if any of the inputs thereof are a binary ZEROindicating that one or more software interrupts are pending. The fouroutput signals of software interrupt encoder 257-1 are connected to thefour input signals of the I3 input to major branch multiplexer 256-1 andare gated onto the four outputs thereof, signals RAMBT2+ through RAMBT5+when multiplexer select signals RDDT41+ through RDDT43+ equal a binary011 (see FIG. 35D). These four output signals a major branch multiplexer256-1 are in turn connected to four of the ten inputs to the I2 inputsof address generator 1 multiplexer 248-1 thereby allowing the CPUfirmware to test for the presence of a pending software interrupt byspecifying the appropriate multiple test branch within the firmwaremicroinstruction. As seen above in discussing FIG. 34, the CPU firmwareis coded such that the presence of a pending software interrupt istested at the beginning of the execution of each software instructionthereby allowing the CPU firmware to branch to the microroutine coded tohandle the particular software interrupt.

Boot PROM Logic

As described above in the discussion of the system startup andinitialization sequence in conjunction with FIG. 16, the system has theability to read a software program stored in boot PROM and load it intomain memory for execution from main memory. Referring now to FIG. 42, itcan be seen that 8 bits of the 48-bit CPU firmware microinstruction wordare contained in parallel PROM memories. Specifically, bits 24 through31 of the 48 bit microinstruction word may be read from either boot PROM240 or ROS 2, 238-2. As described above, normally these eight bits areread from ROS 2, 238-2 when ROS 2 is enabled by both signals KENROS- andPENBBT+ being a binary ZERO. Alternatively, during systems start up,signal PENBBT-, the input of inverter 589, may be set to a binary ZEROby encoding a binary 0010 in bits 24 through 27 of the firmware word anda binary ZERO in bit 32 (see FIGS. 35C and 37F). Microinstruction wordbits 24 through 27 and bit 32 are decoded by a decoder which is enabledby the clock signal PTIME0+ at an inverted enabling input. Therefore,signal PENBBT- will be a binary ZERO from the beginning of primary time1 through the end of primary time 4.

If the firmware microinstruction indicates that the boot PROM should beread, signal PENDBT- being a binary ZERO will partially enable boot PROM240 and, if no clear operation is in progress, signal PCPCLR+ will alsobe a binary ZERO fully enabling boot PROM 240 allowing the 8 bitsthereof, signals RROS24+BT through RROS31+BT to be output. With thesignal PENBBT- being a binary ZERO the output of inverter 589 will be abinary ONE disabling ROS2, 238-2, causing no data to be read therefrom.The tri-state outputs of boot PROM 240 and ROS 2, 238-2, are wire ORedtogether at point 599 producing signals RROS24+ through RROS31+ which inturn are clocked into local register 242 at the beginning of primarytime 0 by clocking signal PTIME0+. By examining FIG. 42 it can be seenthat if a current CPU firmware microinstruction enables boot PROM 240,bits 24 through 31 of the next microinstruction read from the controlstore will be taken from boot PROM 240 as opposed to being taken fromROS 2, 238-2. The address inputs to boot PROM 240, signals PRFR15+through PRFR06+ , are the 10 least significant bits from the function(F) register. Therefore, as will be seen below, the CPU firmware has theability to load an address into the function (F) register and controlthe incrementing or decrementing of the F register along with othernecessary control required to allow the CPU firmware to access the bytesof data stored in boot PROM 240 and store them into main memory.

Before leaving the discussion of the boot PROM logic it should be notedby referring to FIG. 35C that the 8 bits which are read from boot PROMmemory are substituted into the 8 bits which would otherwise be used tohold a constant when the subcommand decode field, bits 32 through 34,equals a binary 111. This allows those microinstructions in which the 8bits from boot PROM 240 are substituted for the 8 bits from ROS 2,238-2, to still perform most of the microinstruction had there been nosubstitution of the 8 bits from the boot PROM for the 8 bits from ROS 2.

For simplicity, in FIG. 42, local register 242 is shown as one registercontaining 48 bits. In the preferred embodiment, local register 242 isimplemented by six 8-bit registers. For simplicity in FIG. 42 the reset(R) input to local register 242 is shown as being a binary ONE therebyindicating that local register 242 is never cleared. In the preferredembodiment, the reset input of the 8-bit register which output signalsRDDT16+ through RDDT23+ has as its reset (R) input the signal PCPCLF-which allows these 8 bits to be cleared during system initialization.

CPU LOGIC DETAILS

FIG. 43 shows the CPU logic described above in conjunction with FIGS. 8and 12.

Internal bus 260 consists of 16 lines, signals PBUS00+ through PBUS15+,which are used to carry addresses and data throughout the CPU. Thetri-state outputs of data transceiver A, 288-1, data transceiver B,284-1, and microprocessor 232 are wire ORed together at point 509. Datatransceiver A and B, 288-1 and 284-1, are each composed of two 8-bit bustransceivers of the type SN74LS245 manufactured by Texas InstrumentsIncorporated of Dallas, Texas. Depending upon the status of thedirection (DIR) input, data transceivers A and B either receive datafrom system bus A and system bus B and place it on internal bus 260 ortake the data from internal bus 260 and transmit it to system bus A andsystem bus B. If the signal at the DIR input of data transceiver A ordata transceiver B is a binary ZERO the signals at the D inputs areplaced on the Q outputs (i.e., data is received from the system bus). Ifthe signal at the DIR input is a binary ONE, the signals at the Q inputsare placed on the D outputs (i.e., the data on the internal bus istransmitted to the system bus).

Data Transceiver Logic

As discussed above, the transmission or reception of data by datatransceiver A and data transceiver B is controlled separately by the CPUfirmware. Therefore, when a main memory on system bus B is providingdata to the CPU or an I/O controller which may be located on eithersystem bus A or system bus B, data transceiver B will be set to receivethe data from the system bus via signals BUSB00+ through BUSB15+ andpass it to the internal bus and data transceiver A will be controlled totake the information from the internal bus and place it onto system busA via signals BUSA00+ through BUSA15+. When data is being sent by theCPU to either the main memory on system bus B or an I/O controller onsystem bus A or B, both data transceiver A and B will be controlled totake the information from the internal bus and transmit it to system busA and system bus B. If an IOC on either system bus A or B is sendinginformation to the CPU or main memory, CPU firmware will control thedata transceiver A and data transceiver B such that the data transceiverassociated with the system bus on which the send IOC is located willreceive the information from the system bus and place it on the internalbus and the other data receiver will take the information from theinternal bus and place it on its associated system bus. By controllingdata transceiver A and data transceiver B, 288-1 and 284-1, in thismanner, the data appearing on the 16 address/data lines on the systembuses is always the same.

Looking now at data transceiver A, 288-1, the direction (DIR) input,signal PBSFAP- is the output of NOR gate 512. One input of NOR gate 512is signal PBSFKP- which is a binary ZERO when bits 24 through 27 of themicroinstruction word are a binary 0101 (see FIG. 37F) which is decodedby a decoder (not shown) with signal PTIME0+ at an inverted enablinginput. The other input to NOR gate 512, signal PENBSA-00, is derivedfrom an I/O command 1 decoder, 244-3, when bits 24 through 26 equalbinary 100 (see FIG. 37C) via NAND gate 521. Turning now to datatransceiver B, 284-1, the direction (DIR) input, signal PBSFBP- is theoutput of NOR gate 510. One input of NOR gate 510 is signal PBSFMD-which is one of the outputs of interrupt request register 250-1 (seeFIG. 42) which is loaded with signal PMRCYC-, the output of NAND gate531. The other input to NOR gate 510 is signal PENBSB-00 which is alsoderived from I/O command 1 decoder 244-3, via NAND gate 534 when bits 24through 26 equal a binary 110 (see FIG. 37C).

Data on internal bus 260 may be loaded into channel number register 296and from there into scratch pad memory address multiplexer 294, theoutput of which is used to address scratch pad memory 236.Alternatively, the data on internal bus 260 may be loaded into byteswapping multiplexer 262, the output of which may be written intoscratch pad memory 236. In addition, data on the internal bus 260 may beloaded into data select multiplexer 269, the output of which may beloaded into the F register 274, I register 270, M1 register 272 andmicroprocessor 232.

Scratch Pad Memory Logic

Channel number register 296 will be loaded with the seven leastsignificant bits of the 10-bit channel number by clocking internal bus260 data lines PBUS03+ through PBUS09+ onto the data inputs thereof whenthe clocking signal PCLKCH- transitions from the binary ZERO to thebinary ONE state. Because the channel number register is used inconjunction with DMC I/O controllers to address the PCT table in scratchpad memory 236, only the 7 least significant bits of the channel numberneed to be saved in the channel number register (see FIG. 24). The threemost significant bits of the DMC channel numbers are always zero.

The state of signal PBYTEX+ from system bus A and B is also input tochannel number register 296 and output as signal PDMCIO+. Channel numberregister 296 clocking signal PCLKCH- is derived from NAND gate 504. Oneinput of NAND gate 504, signal PIOCTL+ is output by inverter 528 whichinverts the output of subcommand decoder 244-2 which decodes the statusof firmware word bits 32 through 34 (see FIG. 35C). The two other inputsto NAND gate 504 are signals RDDT27+ and RDDT28+ which are output bylocal register 242 (see FIG. 42) and when both set equal to a binary ONE(see FIG. 37D) fully enable NAND gate 504 making the output thereof,signal PSAFBS-, a binary ZERO. When signal PSAFBS- is a binary ZERO theoutput of inverter 506, signal PSAFBS+, will be a binary ONE partiallyenabling AND gate 508. AND gate 508 will be fully enabled when thesignal PTIME3+ transitions from the binary ZERO to the binary ONE stateat the beginning of primary time 3 thereby causing the clocking signalPCLKCH- to transition from the binary ZERO to the binary ONE state andclock channel number register 296.

The outputs of channel number register, signals PSAR01+ through PSAR06+,are inputted to the I1 inputs of SMP address multiplexer 294. The othertwo outputs of channel number register 296, signals PDMCIO+ and signalPSAIOB+, are input to the branch on test network 254 (FIG. 9) the outputof which, signal RASBT9+ is input into address generator 1 multiplexer,248-1, (FIG. 42). Signal RASBT9+ is used by the CPU firmware to testwhether the IOC type is a DMA IOC or a DMC IOC when derived from signalPDMCIO+, or whether the input channel or output channel is being usedwhen signal RASBT9 is derived from signal PSAIOB+.

In addition to clocking channel number register 296, signal PCLKCH- alsoclocks address/range flip-flop 502. Clocking signal PCLKCH- clocks theRDDT19+ signal at the data (D) input of address/range flip-flop 502. Bycontrolling the state of bit 19 of the CPU firmware word and theclocking of the address/range flip-flop 502, the CPU firmware cancontrol the setting or resetting of flip-flop 502. The setting andresetting of the address/range flip-flop 502 are also controlled bysignals PSAR7S- and PSAR7R- at the set (S) and reset (R) inputs thereof,which are in turn output from a decode of firmware word bits 24 through26.

The output of address/range flip-flop 502, signal PSAR07+ is input tothe I1 input of SPM address multiplexer 294 and is used as the leastsignificant bit in addressing scratch pad memory 236. A binary ONE isset at the most significant bit of the I1 inputs of SPM multiplexer 294and when the I1 inputs are selected by signal RDDT22+ being a binary ONEat the select (SEL) input of SPM address multiplexer 294 the programchannel table located in the scratch pad memory 236 is accessed. Whenaddressing the PCT in SPM, the least significant bit of the SPM addressdetermines whether the address word or the range word of the PCT entryaddress by the 6 bits from channel number register 296 is referenced.That is, if address/range flip-flop 502 is set, causing the outputthereof (signal PSAR07+) to be a binary ONE, the range word of a PCTchannel pair is addressed and if flip-flop 502 is reset, the addressword of the PCT channel pair entry is addressed (see FIG. 10).

The IO inputs of SPM address multiplexer 294 have their four mostsignificant bits set equal to a binary ZERO and the other four bits aresignals RDDT01+ and RDDT05+ through RDDT07+. This allows four bits fromthe microinstruction CPU firmware word to address the SPM 236 when theIO inputs are selected by signal RDDT22+ being a binary ZERO (see FIGS.35A and 35C). The outputs of SPM address multiplexer 294 are enabledwhen the signal KENSPA- at the output control (F) input thereof is abinary ZERO.

Scratch pad memory 236 receives its 8-bit address, signals PSARS0+through PSARS7+, from the SPM address multiplexer 294. Scratch padmemory 236 is composed of 17 1-bit by 256 RAM chips of the type 5539-2manufactured by Intersil Incorporated of Cupertino, California, anddescribed in their publication entitled "Intersil Semiconductor ProductsCatalog," incorporated herein by reference. The RAM chips are arrangedso that SPM 236 provides 256 words of 17-bits each as shown in FIG. 10.Writing into SPM 236 is enabled by placing a binary ZERO at the writeenable (WE) input of the scratch pad memory. If bit 0 of the firmwareword is a binary ONE, signal RDDT00+ will be a binary ONE partiallyenabling NAND gate 587. NAND gate 587 will be fully enabled when clocksignal PTIME4+ transitions from a binary ZERO to the binary ONE state atthe beginning of primary time 4 and cause the output of NAND gate 587,signal PSDWRT-, to become a binary ZERO.

The 16 most significant bits of the data input to SPM 236 are signalsPSDI00- through PSDI15- from the output of byte swapping multiplexer262. The least significant bit of the data input signals to SPM 236 issignal RDDT19+ which is derived from bit 19 of the firmware word. Theoutput of SPM 236 is signals PSDO00+ through PSDO16+. SPM data register518 is composed of two 8-bit D-type transparent latches of the typeSN74S373 manufactured by Texas Instruments Incorporated of Dallas,Texas. The inputs of SPM data register 518 are enabled onto the outputsthereof during primary time 0 and primary time 1 via signals PTIME0- andPTIME1- at OR gate 585. The output of OR gate 585, signal PSDRCK+, isconnected to the clock (C) input of SPM data register 518. The output ofSPM data register 518 is always enabled by a binary ZERO appearing atthe output control (F) input thereof. The outputs of SPM data register518, signals PSDR00+ through PSDR15+ are connected to the I0 inputs ofdata selecter multiplexer 269.

Clear flip-flop 514 is set when the clocking signal PCLEAR- transitionsfrom the binary ZERO to the binary ONE state and can be reset by thesignal PCLERR- becoming a binary ZERO at the reset (R) input thereof.The Q output of flip-flop 514, signal PCPCLF+, and signal PSD016+, whichcontains the least significant bit of the 17-bit word read from thescratch pad memory 236, are clocked into register 516 when the clockingsignal PTIME4+ transitions from the binary ZERO to the binary ONE stateat the beginning of primary time 4. The other output of clear flip-flop514, signal PCPCLF-, is used to clear bits 16 through 23 of themicroinstruction firmware word contained in local register 242 (see FIG.42).

Signal PCPCLR+, from register 516, and signal PCPCLR-, from inverter583, are used throughout the CPU to clear various registers andflip-flops. For example, signal PCPCLR+ is used to clear addressregister 1, 246-1, via NOR gate 595 and signal PCPCLR- is used to clearaddress register 2, 246-2, (see FIG. 42). Signal PCPCLR+ is also used todisable the reading of data from boot PROM 240 (see FIG. 42). The otheroutput shown on register 516, signal PTBS13+ which reflects the state ofthe least significant bit of the word read from the scratch pad memory236, goes to branch on test network 254 (FIG. 9) and is used todetermine bit RASBT9+ which is input into address generator 1multiplexer 248-1 (FIG. 42).

Byte Swapping Logic

Turning now to byte swapping multiplexer 262, in FIG. 43, the manner inwhich the CPU firmware can control the swapping of the left and rightbytes of the data on the internal bus prior to writing it into scratchpad memory 236 will be described. Subcommand 1 decoder 244-1 is used todecode bits 25 through 27 of the microinstruction firmware word and whenset equal to binary 001 (see FIG. 37F) cause one of the outputs thereof,signal PSDEXEN-, to be set equal to a binary ZERO. The output of thesubcommand 1 decoder 244-1 is partially enabled by signal RDDT24- beinga binary ONE and signal RDDT32+ being a binary ZERO and is fully enabledby signal PTIME0+ transitioning from a binary ONE to a binary ZERO stateat the beginning of primary time 1 and staying in the binary ZERO stateuntil the end of primary time 4. When signal PSDEXEN- is a binary ZEROthe I0 inputs of byte swapping multiplexer 262 are enabled onto theoutputs thereof causing the interchange of the left and right bytesappearing on internal bus 260. If signal PSDEXEN- is a binary ONE, theI1 inputs of byte swapping multiplexer 262 are selected and the data onthe internal bus is transferred to the outputs of multiplexer 262without swapping the bytes. In this manner the swapping of the bytes ofthe data from the internal bus 260 may be controlled prior to the databeing written into the scratch pad memory 236.

Microprocessor and Data Selection Logic

As described above, the third input to internal bus 260 is from the dataoutput by microprocessor 232. Microprocessor 232 receives 16 bits ofinput data, signals PAUD15+ through PAUD00+, from the output of dataselecter multiplexer 269. The microprocessor register file is addressedby bits PSPA03+ through PSPA01+ and bit RDDT04+. File register addressbits PSPA03+ through PSPA01+ are derived from file address multiplexer276 shown in FIG. 8. The file address multiplexer selection iscontrolled by bits 2 and 3 of the firmware word (see FIG. 35A). Theinstruction performed by the microprocessor 232 is controlled by the 9instruction (INSTR) inputs, signals RDDT10+ through RDDT18+, which areobtained from the firmware word bits 10 through 18 (see FIG. 35B). Asdescribed above, the data output of microprocessor 232, signalsPBUS15+CP through PBUS00+CP are wire ORed with the output of datatransceiver A, 288-1, and data transceiver B, 284-1, at point 509.

The I1 inputs of data selecter multiplexer 269 are derived from the 8bits of indicator (I) register flip-flop 270, signals PI08+ throughPI15+, and the outputs of mask (M1) register 272, signals PRMR00+through PRMR07+. The I2 inputs of data selecter multiplexer 269 aresignals RDDT08+, RDDT09+ and RDDT24+ through RDDT31+. The output of dataselecter multiplexer 269 is selected from the four inputs by signalsRDDT20+ and RDDT21+ at the two selecter (SEL) inputs thereof. The binaryZERO at the strobe (CE) input of data selecter multiplexer 269 enablesthe outputs. By controlling firmware word bits 20 and 21 the output ofdata selecter multiplexer 269 may be selected from: scratch pad memorydata register 518; I register 270 and M1 register 272; a constant frommicroinstruction word bits 8, 9 and 24 through 31; and the internal bus260 (see FIG. 35C).

I, M1 and F Register Logic

The indicator (I) register 270 is composed of eight D-type flip-flops,each of which is clocked by signal PIRFBS- when it transitions from abinary ZERO to the binary ONE state. Signal PIRFBS- is generated bydecoding microinstruction firmware bits 24 through 27 which, when theyequal a binary 110, will set signal PIRFBS- to a binary ZERO (see FIG.37F). Mask (M1) register 272 is clocked by signal PRMRCK- transitioningfrom a binary ZERO to a binary ONE state. Signal PRMRCK- is generated bydecoding bits 28 through 31 of the microinstruction firmware word which,when equaled to a binary 1100, will set the signal to the binary ZEROstate (see FIG. 37G). M1 register 272 will be cleared by signal PCPCLR-being set to a binary ZERO. It should be noted that clocking signalsPIRFBS- and PRMRCK- are generated by decoders which are enabled at aninverting enabling input by signal PTIME4-; therefore both of theseclocking signals will transition from the binary ZERO to the binary ONEstate at the end of primary time 4.

The function (F) register is comprised of F register counter FR0, 274-0,F register counter FR2, 274-2, and F register counter FR3, 274-3. FR0,274-0, and FR3, 274-3, are each comprised of one type SN74LS169Asynchronous 4-bit up/down counter manufactured by Texas InstrumentsIncorporated of Dallas, Texas. FR2, 274-2, is comprised of two typeSN74LS169A counters. The clocking of FR0, FR2 and FR3 is controlled bythe signal PRFCLK+ at the clock (C) input thereof. Signal PFRCLK+ willtransition from a binary ZERO to the binary ONE state at the end ofprimary time 3 if microinstruction firmware word bits 32 through 34equal a binary 100 (see FIG. 35C). The loading of F register counterFR0, 274-0, is controlled by signal PFRLD0- at the load (L) inputthereof. Signal PFRLD0- is produced by NANDing together signals RDDT26+and RDDT27+. The loading of F register counter FR2, 274-2, is controlledby signal PRFLDM- at the load (L) input thereof. Signal PFRLDM- isproduced by NANDing together signals RDDT28- and RDDT29+. The loading ofF register counter FR3 with input signals PAUD15+ through PAUD12+ iscontrolled by signal PFRLD3- at the load (L) input thereof. SignalPFRLD3- is produced by NANDing together signals RDDT30- and RDDT31+.

F register counter FR0, 274-0, is enabled to count by signal RDDT26+ atthe count enable (P and T) inputs thereof. F register FR2, 274-2, isenabled to count by signal RDDT28+ at the count enable (P and T) inputsthereof. F register counter FR3, 274-3, is enabled to count by signalRDDT30+ at the count enable (P and T) inputs thereof. The direction ofcount of F register counter FR0, 274-0, is controlled by signal RDDT27+at the up/down (U/D) input thereof. F register counter FR2, 274-2,direction of count is controlled by signal RDDT29+ at the up/down (U/D)input thereof. The direction of count of F register counter FR3, 274-3,is controlled by signal RDDT31+ at the up/down (U/D) input thereof. Someof the outputs of the F register are inputs of the boot PROM 240 (FIG.42), file address multiplexer 276 (FIG. 8), and the branch on testnetwork 254 (FIG. 9). The outputs of the F register are also used inother places within the CPU logic.

Bus Command Logic

The varous command signals which go from the CPU to the I/O controllersand the main memory on system bus A and B are illustrated in FIG. 43.The signals originating from command transceiver A, 286-1, and commandtransceiver B, 282-1, and signals PIOCTA- and PIOCTB- all originate inthe CPU and are transmitted on the system buses to the I/O controllersor main memory. Other command signals such as PBYTEX-, PBUSY-2A andPBUSY-2B, etc., can originate in the CPU and be sent to the I/Ocontrollers or main memory or may originate in an I/O controller and besent to the CPU via the system buses.

I/O Command Logic

Referring now to FIG. 43, command transceiver A, 286-1, and commandtransceiver B, 282-1, are of the type SN74LS245 manufactured by TexasInstruments Inc., of Dallas, Texas, and are as discussed above withrespect to data transceiver A, 288-1, and data transceiver B, 284-1. Byhaving binary ZEROs at the direction (DIR) and output enable (F) inputsof command transceiver A and command transceiver B, both transceiversare conditioned to always having their outputs enabled and the directionof information transfer is from the CPU (D) inputs to the system bus Aand B (Q) outputs. Input signals RDDT29+ through RDDT31+ are derivedfrom firmware word bits 29 through 31 and are used to send I/O commands(see FIG. 37E) to the I/O controllers on system buses A and B and arecommon to command transceiver A and command transceiver B, 286-1 and282-1. Signals BCYCOT- and PTIME3- are timing signals derived from theclock logic shown in FIG. 14 and are transmitted on both system bus Aand system bus B by command transceiver A, 286-1, and commandtransceiver B, 282-1. Initialization signal PCLEAR- is likewisetransmitted on both system bus A and system bus B. Enable IOC datadriver signal PENBSA-00 is the output of NOR gate 534 and aretransmitted to system bus A and system bus B by command transceiver A286-1, and command transceiver B, 282-1, respectively.

Subcommand decoder 244-2 is enabled at the beginning of primary time 1and remains enabled through primary time 4 by signal PTIME0+ at theenable (EN) input. Subcommand decoder 244-2 decodes signals RDDT32+through RDDT34+ and produces signals PMRFSH-, PMRCON- as describedabove, and signal PIOCTL (see FIG. 35C). Signal PIOCTL is inverted byinverter 528 to produce signal PIOCTL+. Signal PIOCTL+ partially enablesNAND gate 536 and NAND gate 538. When NAND gate 536 is fully enabled bysignal RDDT27- being a binary ZERO, the output thereof, signal PIOCTA-,is a binary ZERO indicating to the IOCs on system bus A that the I/Ocommand encoded on lines RDDT29+A through RDDT31+BA is valid. In asimilar manner, NAND gate 538 is fully enabled by signal RDDT28- being abinary ZERO. This makes the output of NAND gate 538, signal PIOCTB-, abinary ZERO indicating to IOCs on system bus B that an I/O commandencoded on lines RDDT29+BB through RDDT31+BB is valid (see FIG. 37D).

Subcommand decoder 244-2 output signal PIOCTL- is also used to enableI/O command 1 decoder, 244-3, which decodes signals RDDT24+ throughRDDT26+ (see FIG. 37C). I/O command 1 decoder, 244-3, output signalsPBSIOA- and PBSIOB- are each connected to NAND gate 521 and NAND gate534 respectively, partially enabling the respective NAND gates when thedecode of bits 24 through 26 of the microinstruction firmware word setsthe respective output signals to a binary ZERO. NAND gates 521 and 534are fully enabled by signals PBSFMD+ being a binary ZERO. Signal PBSFMD+is the output of inverter 520 which inverts signal PBSFMD- originatingfrom interrupt request register 250-1 (see FIG. 42).

Proceed and Busy Logic

Output signal PROCED- from I/O command 1 decoder, 244-3, is inverted byinverter 544 to produce signal PROCED+ which is in turn inverted byinverters 546 to produce signal PROCED-2A and inverter 547 to producesignal PROCED-2B. Signal PROCED-2A goes to system bus A and signalPROCED-2B goes to system bus B to indicate to I/O controllers on therespective buses that the CPU has accepted the last interrupt requestfrom an IOC on the respective system buses. Alternatively, the CPU mayreceive a proceed signal from an IOC on system bus A on line PROCED-2Aand by an IOC on system bus B on line PROCED-2B. Both of these lines areinput to NOR gate 550 which produces signal PROCED-20 which in turn goesto branch on test network 254 (see FIG. 9) which produces signal RASBT9+used by address generator 1 multiplexer 248-1 (see FIG. 42) therebyallowing the CPU firmware to test whether the addressed IOC on systembus A or B has accepted the I/O command on system bus lines RDDT29+through RDDT31+.

Signal PBBUSY-, at the output of I/O command 1 decoder, 244-3, goes intologic similar to that of the proceed logic described immediately aboveto produce and receive busy signals to and from I/O controllers onsystem bus A and B. Specifically, signal PBBUSY- is inverted by inverter552 to produce signal PBBUSY+ which is in turn inverted by inverter 554to produce signal PBUSY-2A which goes to or comes from system bus A andinverter 556 which produces signal PBUSY-2B which goes to or comes fromsystem bus B. Signals PBUSY-2A and PBUSY-2B are input into NOR gate 558which produces signal PBBUSY-20 which goes to branch on test network 254(see FIG. 9) the output of which, signal RASBT9+ goes to addressgenerator 1 multiplexer, 248-1 (see FIG. 42).

Read/Write Byte Logic

Write byte 0 and write byte 1 signals, PWRTB0+ and PWRTB1+, canoriginate from the CPU by inverting RDDT08+ through inverter 524 andRDDT09+ through inverter 526 respectively or they may be received froman IOC located on system bus A or system bus B. Therefore, the writebyte signals may be controlled by bits 8 and bits 9 of the firmware word(see FIG. 35B) or received from an IOC on one of the system buses.Signals PWRTB0+ and PWRTB1+ are both used to partially enable AND gate530 whch produces output signal PMREAD+. Signal PMREAD+, when a binaryONE, partially enables NAND gate 531 which will be fully enabled whensignal PMMCYC+ output by AND gate 564 is a binary ONE. The output ofNAND gate 531, signal PMRCYC-, goes to one of the inputs of theinterrupt request register 250-1 (see FIG. 42).

Memory Go Logic

The main memory go signal, which informs the memory to perform a read,write or refresh cycle, is primarily generated by controlling bits 23 ofthe firmware word (see FIG. 35C). Conditional read signal PMRCON- is oneof the outputs of subcommand decoder 244-2 and is inverted by inverter560 to produce signal PMRCON+ which partially enables NAND gate 562.Control flop 4 flip-flop 258-4 is set by signal PTCF4S- being a binaryZERO at the set (S) input thereof and can be reset by the signal PTCF4R-being a binary ZERO at the reset (R) input thereof. Control flop 4flip-flop 258-4 can also be set by gating the most significant bit ofthe 16-bit internal bus 260 appearing on line PBUS00+ at the data (D)input thereof by clocking signal PCTF4C- transitioning from the binaryZERO to the binary ONE state. Signals PTCF4S-, PTCF4C- and PTCF4R- areproduced by decoding bits 28 through 31 of the firmware word (see FIG.37G). When set, the output of control flop 4 flip-flop 258-4, signalPTCF04+, will be a binary ONE fully enabling NAND gate 562 and settingsignal PRMCON-20 at the output thereof to the binary ZERO state. Bydoing a conditional read, signal PMRCON- being a binary ZERO, and bysetting control flop 4 flip-flop 258-4, signal PTCF04+ being a binaryONE, signal PMRCON-20 will be a binary ZERO and disable AND gate 564.Disabling AND gate 564 will inhibit the generation of the memory gosignal which would otherwise occur if bit 23 of the microinstruction isa binary ONE.

This ability to inhibit the generation of a memory go signal is used bythe CPU firmware in those situations (such as in processing store typesoftware instructions) in which it is not desired to do a memory readprior to doing a store into the same main memory location. In thesesituations, the main memory read (which might detect a memory parityerror) is inhibited and only the memory write operation is permitted(which does not check for parity errors) to eliminate the possibility ofa memory parity error trap.

If signal RDDT23+ is a binary ONE and signal PMRCON-20 is a binary ONE,AND gate 564 will be fully enabled and the output signal PMMCYC+ will bea binary ONE at the data (D) input of memory go flip-flop 566. A binaryONE at the data input of memory go flip-flop 566 will be clocked bysignal PTIME3+ transitioning from the binary ZERO to the binary ONEstate at the beginning of primary time 3 thereby setting the flip-flopand making output signal PMEMGO+CP a binary ONE. Memory go flip-flop 566will be reset at the beginning of primary time 0 when signal PTIME0-transitions from a binary ONE to the binary ZERO state. Signal KMEMGO-is inverted by inverter 568 producing signal PMEMGO+MP. If eithersignals PMEMGO+CP or PMEMGO+MP at the inputs of NOR gate 570 are abinary ONE, the output signal PMEMGO- will be a binary ZERO and signalthe memory on system bus B to begin a main memory read, write or refreshcycle.

MAIN MEMORY DESCRIPTION

Main memory is a high-speed, Random Access Memory (RAM) that is capableof performing all read/write functions without restrictions on addresssequence, data patterns, or memory repetition rates. Its basicarchitecture consists of a unique memory configuration designed toprovide a minimum to maximum, read/write storage (RAM) of 16K to 64K16-bit words, respectively. Also included are two parity bits per word.The main memory performance characteristics consist of memory read,write, or refresh cycles of 1000 nanoseconds each (i.e., two consecutive500 nanosecond CPU cycles), a memory read access time of 500nanoseconds, and a memory refresh rate of 15 microseconds.

SYSTEM BUS INTERFACE

All transfer of information (i.e., addresses, data, or CPU commands)between main memory and the CPU, or between main memory with I/Ocontrollers, is over system bus B. Main memory control is provided bymemory commands sent exclusively by the CPU. The interface signalsbetween main memory and the system bus are: BUSX00+ through BUSX15+,PTIME3-, PCLEAR-, PWRTB0+, PWRTB1+, MEMPER-, PMEMGO-, PMFRSH- andPBSFMD-, and are shown in FIGS. 6 and 7.

Data Word

The main memory data word consists of 16 data bits (2 bytes) as shown inFIG. 5 with 2 parity bits (1 parity bit per byte). Because the 2 paritybits are unique to the main memory, they are not transmitted over thesystem bus to the CPU or IOC's.

Address Word

The memory address word (see FIG. 5) permits software to access a memorylocation in a main memory module within a main memory configuration.

MAIN MEMORY ORGANIZATIONAL OVERVIEW

A main memory configuration is organized as a read/write memory (RAM) byutilizing one or more main memory boards. A main memory board provides16K, 32K, or 64K of read/write storage. The minimum to maximum storagecapacity for a main memory configuration is 16K to 64K 16-bit words(16KW to 64KW), respectively. The main memory address field, set inaddress switches on the one or more main memory boards must: (1) becontiguous from location 0 in a main memory configuration to the end ofthe memory configuration; (2) always start on a 4KW address boundary;and (3) always maintain the largest size main memory board (or module)in low-order memory (0-X), with each additional smaller main memoryboard starting at the last address (X), plus one (i.e., X+1), of itspreceding larger main memory board (or module).

MODULE PHYSICAL/ORGANIZATIONAL CHARACTERISTICS

The main memory boards utilize 16K by 1-bit RAM chips to form a uniqueRAM data and parity array on each main memory board for addressing easeand storage versatility. The array is organized into four rows (0through 3). Each row utilizes 18 RAM chips that are arranged to provide16K 16-bit words with 2 parity bits per word within each row. Each mainmemory board can be configured to provide 16KW, 32KW, or 64KW ofread/write storage capacity by populating each row accordingly. Thestorage capacity on an assembled main memory module can be determined bythe number of rows populated with the 16KW RAM chip (e.g., population ofrows 0 and 1 on a main memory board equals 32KW of storage).

Module Addressing

An address switch assembly is used to assign a main memory board with afixed address range in a main memory configuration and to indicate theamount of memory on the main memory board.

The address switch assemblies are used exclusively to form a main memoryaddress within a main memory module. This address is compared with theone in the segment identifier field (bits 0 through 3) from the systembus address lines to determine the main memory module being addressed.Each module size switch in the address switch assembly on the mainmemory module enables a 4KW main memory address field when placed in itsOFF position, thus enabling manual formation of a 16KW to 64KW mainmemory size in 4K increments.

MEMORY SAVE UNIT

The memory save unit (element 222 in FIG. 1) is used with the mainmemory boards. The memory save unit maintains dc power on the RAM arrayon the main memory boards when ac source power to the system isunexpectedly interrupted and, as long as the memory save unit remainsactive, enables continuous refresh cycles until the ac source power isrestored.

MAIN MEMORY FUNCTIONAL OVERVIEW

This section provides a functional overview of main memory to highlightits activity in the processing of data from the system bus.

FIG. 41 illustrates the major elements in a main memory module and showsthe data flow activity between main memory and the system bus. Controlinformation (i.e., refresh commands, read/write commands, address, etc.)are fetched from the system bus by main memory and temporarily storedinto pertinent memory registers to process the data, or to refresh mainmemory as needed. The applicable activity is implemented when a memorycycle is initiated by a memory start request from the CPU (signalPMEMGO- becoming a binary ZERO).

When a refresh command is received, a selected row in each RAM chip maybe refreshed as described in subsequent paragraphs. When a read or writecommand is received, the memory address is first tested to verify thatit does not exceed the prescribed address boundaries (i.e., after thestarting address and within the starting address plus the memory size).If the boundaries are exceeded, the memory cycle is terminated. If theyare not exceeded, the memory cycle continues to process the request byexamining the command register for a read or write command. Receipt of aread command initiates a memory read cycle to fetch a 2-byte data wordfrom a main memory location to the system bus. Similarly, receipt of awrite command for a word or byte memory operation initiates a memorywrite cycle to fetch a data word from the system bus and to store theword, or either byte from the word, into a prescribed main memorylocation. The CPU enables the system bus to accept data for a memoryread operation, and ensures that data is on the system bus for a memorywrite operation.

In a write operation, a 2-byte data word is fetched from the system busand stored into a memory data-in register. Then, depending upon the typeof write function (word or byte write), a word or byte write memoryoperation is performed to store the word or byte in memory, and a paritybit is generated and stored for each byte (i.e., 2 parity bits per word,or 1 parity bit per byte). In a byte operation, when byte 0 (left) orbyte 1 (right) is stored into a memory location, the adjacent location(opposite) byte of the memory location is unaffected.

In a read cycle a 2-byte data word, along with its 2 parity bits, isfetched from a prescribed memory location. The 2 parity bits are sentdirectly to the parity check logic, while the 16-bit data word is sentdirectly to the data out register where it is placed on the system busfor acquisition by the CPU. This data word is also recirculated from thedata out register, through the data in register, and to the parity checklogic where, along with the 2 parity bits, it is checked for a possibleparity error. When a parity error occurs, it is latched up in a memoryerror latch, an error light on the defective main memory board isilluminated, and the CPU is notified accordingly (over the system bus online MEMPER-).

When main memory receives and accepts a refresh command (sent by the CPUevery 15 microseconds or less), data to the system bus is inhibited, anda read cycle is initiated to restore data in the selected locations inthe RAM array on each main memory board. Upon completion, a refreshaddress counter is incremented by one in preparation for the nextrefresh command.

Main Memory Timing

A main memory operation utilizes two consecutive 500-nanosecond CPUcycles that coincide with a 1000-nanosecond memory cycle. The first CPUcycle requests a memory cycle (memory go) and provides a memory addressand the respective commands. The second CPU cycle provides system busdata for, or receives system bus data from, a prescribed memory address.

In the first CPU cycle, main memory receives a refresh or a read/writecommand, and a low-level memory go (binary ZERO PMEMGO-) signal. Therefresh or read/write command conditions a main memory board for theapplicable operation. However, it should be noted that a low-levelrefresh command signal (PMFRSH-) is placed on the system busapproximately 200 nanoseconds before a memory cycle is requested toalert main memory of the forthcoming refresh command.

The memory go signal (PMEMGO-) is distributed to each main memory modulewithin the main memory configuration, and starts the clock generator ineach main memory board. If a refresh command is received, the memoryread cycle is initiated for a full memory cycle to refresh one row ofeach RAM chip within the array. If a read/write command is received, thefour most significant bits (0-3) of the memory address from the systembus are compared with the segment address in each main memory module. Asuccessful compare in any one main memory module defines it as the onebeing addressed, and also produces a memory present (MMPRES) signal.This signal (MMPRES) initiates a memory cycle for a read or writeoperation within the selected main memory board. If a miscompare occurs,the request is aborted and the memory cycle is terminated.

During the second 500-nanosecond CPU cycle in a write memory operation,data is made available on the system bus from an external source (CPU orI/O controller). In a memory read operation, a low-level enable memorydata signal (PBSFMD-) is received from the system bus to enable memorydata onto the bus. Concurrently, memory also sends a parity error(MEMPER-) signal that when high, indicates an error-free read memoryoperation and when low, indicates that a parity error occurred in thecurrent memory cycle.

Main Memory Modules

This subsection provides a brief functional description of the logicused in main memory modules (boards).

FIG. 41 is a block diagram of main memory boards that shows the signaland the data transfer paths between each element. The key elements arethe RAM data array 630 and RAM parity array 632 (jointly referred to asthe RAM or arrays) used to provide a storage capacity of up to64K-by-16-bit words for a main memory board (2 parity bits per word areincluded). All other elements support the arrays to transfer databetween them and the system bus, and to check or generate parity foreach data word during a read or write memory operation.

Timing Generator

The timing generator 602 provides the timing pulses to activate andsynchronize all operations performed in a main memory module (see FIG.41). It is activated with a request for a memory cycle from the systembus (i.e., a PMEMGO- signal), or with a no prior refresh request(MNOPRR+) signal activated by the power on signal (BPOWON+) when acsource power to the system is disabled. The timing generator 602generates timing signals that are distributed throughout the main memoryboard for clocking address register 604, data in register 606, and writeenable and for strobing the column address and row address.

Negative 5 Volt Generator

The RAM chip requires a -5 Vdc that is generated by a -5-Vdc generator608 located on the main memory board. A +12 Vdc and +5 Vdc from thesystem power supply or memory save unit drive the -5-Vdc generator. Theresulting -5 Vdc from the generator is filtered and routed to the RAMchip in the array.

Power Failure Logic

The power failure logic 628 monitors the bus power on (BPOWON+) signalto warn memory of any forthcoming ac power interruptions. Thismonitoring allows the CPU an additional 2 milliseconds to update memorywith pertinent system information before central processor unitoperation terminates due to ac power failure. After ac power failure,the main memory module is set into an internal refresh mode of operationand cannot be accessed until ac power is restored to the system.

RAM Address Control and Distribution Logic

The address field for a 16KW RAM array is organized into 7 rows and 7columns to form a 128-row-by-128-column matrix. The RAM address controland distribution logic 604 is designed to address a specific location inthis array by generating a specific row and column address for thatlocation. It performs this task with the 16-bit memory address wordstored from the system bus into the data in register 606 at thebeginning of a memory read or write data cycle. Data in register 606 bit3 and bits 10 through 15 form a row address field, and data register bit2 and bits 4 through 9 form a column address field. Both fields areconcurrently stored into their respective row and column addressregisters (634 and 636). The contents of the row address register 634are strobed into the array when a row address strobe signal goes low.The contents of the column address register 636 are then strobed intothe array by a low-level column address strobe signal. Both the row andcolumn addresses remain in the array until overlayed with a new row andcolumn address.

The memory address control and distribution logic 604 also separates andstores a 2-bit memory address field from the data in register 606 into achip select register in chip select logic 624 to select the applicablerow in a main memory module. The specific bits are bits 0 and 1. Thisfield, in conjunction with the 16KW chip functionality in the register,selects row 0, 1, 2, or 3 in a main memory module.

The contents of a refresh row address register in refresh logic 622replace the contents of the row address register 634 during a refreshoperation, and are strobed into the array in the same manner as thecontents from the row address register. In addition, a refresh memorycycle signal is held low during a refresh operation: (1) to refresh anarray by enabling all chips in the applicable array(s), and (2), toprevent the transfer of data from the chip array onto the system bus.

Segment Select Logic

The segment select logic 610 defines the address range of a main memorymodule (using some of the switch settings from address switch assembly638), validates this range with a memory address from the system bus,and implements a memory cycle within the main memory module when theaddress range is not exceeded. If exceeded, a memory cycle is notimplemented. A refresh operation or ac power failure overrides theselect logic to simulate an address present condition and activate therefresh operation in the main memory module.

Data In/Data out Registers

The data in and data out registers (606 and 612) are used as temporarydata buffers for the transfer of data between the system bus (BUSX00+through BUSX15+) and the RAM data array 630 during a write or readmemory operation. They are also used in a read operation to recirculateoutput data to the parity detect logic where this data is checked for apossible parity error.

Parity Generator and Check Logic

The parity generator and check logic 616 monitors the data in registerto generate and store, into the parity array 632, 2 parity bits for each16-bit data word, or 1 parity bit for each data byte during a word orbyte write memory operation. During a read memory operation the paritygenerator and check logic also monitors the data out and parity outregisters (612 and 614) to check parity in each data word.

In a write word operation both parity bits are stored into the parityarray. However, in a write byte operation, only the applicable paritybit for the byte being stored is stored into the parity array. Theadjacent location in the parity array for the opposite parity bit isunaffected.

In a read memory operation a data word from the data in register 606 and2 parity bits from the parity out register 614 are checked in the paritycheck logic for a possible parity error. If a parity error occurs, anerror flop is set in error logic 626 to send a low-level memory parityerror (MEMPER-) signal to the system bus, and to turn on the lightemitting diode (LED) parity error indicator 618 on the main memoryboard.

Read/Write Control Logic

The read/write control logic 620 monitors the system bus for aread/write command code encoded on lines PWRTB0+ and PWRTB1+, decodesthis code (see FIG. 6) for a read (word), write byte 0, write byte 1, orwrite word command, and sends the encoded command to the RAM array forexecution. When a refresh command is received, the read command isautomatically encoded in the read/write control logic 620 to refresh thearray.

Refresh Logic

The purpose of the refresh logic 622 is to activate and control therefresh cycles required to restore data in main (RAM) memory every 2milliseconds (ms). Restoration of RAM data at regular intervals isnecessary because of the dynamic characteristics of a MOS RAM device(e.g., when a MOS RAM device is used as a dynamic memory, deteriorationof its charged data commences within a finite time unless rechargedbefore that time expires). The refresh logic is designed to support arefresh cycle every 15 microseconds to maintain the data in a mainmemory configuration. The 15-microsecond refresh cycle is determined bythe 16K RAM chip array (i.e., 2 milliseconds/128 rows=15 microseconds).

The refresh logic monitors the system bus for a normal refresh command(PMFRSH- being a binary ZERO) to implement a refresh cycle. This commandmust be on the system bus 200 nanoseconds before a memory request(PMEMGO- a binary ZERO) is initiated to alert memory that a refreshcommand is forthcoming. Upon receipt of a memory go signal, the refreshlogic accepts the command, tests it to verify that it has been 15microseconds since the last normal refresh command, and initiates amemory read cycle to implement the command.

The refresh logic also inhibits any data transfer onto the system busthroughout the current refresh cycle. If this is a normal command, thena logical row in every RAM chip is refreshed (recharged) with itsresident data via the current read cycle. Upon completion, a refresh rowaddress counter is incremented by one in preparation for the next normalrefresh command. If a second refresh command is received within 15microseconds, then a read operation is performed at a single memorylocation specified by the memory address register, the data is nottransferred onto the system bus, and the refresh row counter isunaffected.

Chip Select Logic

The chip select logic 624 (also referred to as the start address logic)allows the addition of other main memory modules in a main memoryconfiguration. It performs this function by relocating the main memoryboard address field within the address field of a main memoryconfiguration. It also defines physical row 0 on the main memory boardas the first row of RAM chips. Two switches on the address switchassembly 638 form a 2-bit binary address field used to set one of fourstarting addresses (0K, 16K, 32K, or 48K). The four starting addressesrepresent a logical row position in a main memory configuration (logicalrow 0=16KW, row 1=32KW, row 2=48KW and row 3=64KW).

A more complete explanation of the chip select logic 624 can be found incopending U.S. patent application Ser. No. 921,292 filed July 3, 1978 byChester M. Nibby, Jr. et al. entitled "Rotating Chip Selection Techniqueand Apparatus" and incorporated herein by reference.

MAIN MEMORY SUMMARY

The main memory of the systems is configured by attaching one to fourmain memory modules to system bus B to provide a total of 16K to 64Kwords of main memory. Each main memory module (or board) contains 16K,32K or 64K words. Each word is composed of two 8-bit bytes for data and2 parity bits (1 parity bit per data byte).

All main memory must be located on system bus B which has some signalsunique to main memory that are not found on system bus A. Further, theCPU must implicitly know on which system bus the main memory is whendata is to be read from main memory in order to permit the CPU tocontrol the system buses data line transceivers (i.e., the transceiverfor the system bus on which the data is coming from must be controlledto receive data and the other system bus transceiver must be controlledto transmit the data). By the CPU controlling the system busestransceivers in this manner, the data on both system buses A and B willbe identical and, during the cycle that the data is coming from mainmemory, the CPU need not take into account on which system bus the IOCrequesting the data is located. It should be noted that, during thecycle that data is being sent to main memory, the CPU does take intoaccount on which bus the IOC is located so that the system bustransceivers can be controlled to receive from the system bus on whichthe IOC is located and to transmit to the other system bus, againinsuring that the data (or address) on both system buses is identical.

The MOS main memory is refreshed every 2 milliseconds by logic containedon the main memory board but each row of the RAM chips is only refreshedafter receiving a refresh signal from the CPU. The CPU generates therefresh signal at least once every 15 microseconds (15 microseconds×128rows=2 milliseconds). The refresh operation takes two CPU cycles duringwhich the CPU will not access main memory. By the CPU controlling themain memory refresh in this manner, the CPU (and also all I/Ocontrollers which communicate with main memory under CPU firmwarecontrol) can be guaranteed that the main memory will be available whenan access is attempted. Therefore no system bus signals or recoverylogic must be provided to signal the CPU that the main memory is busyrefreshing and the access must be reattempted. This guaranteed mainmemory availability reduces system bus width and system logicrequirements.

The above description of apparatus for the detection of attempts by theCPU or I/O controllers to access a nonexistent main memory location wasby way of illustration only. It should be understood for example thatthe main memory boards need not be coupled to a common bus to which theCPU and IOC's are also coupled. It should be further understood that theIOC's could all be coupled to one common bus or that they could bedistributed among many common buses. It also should be understood thatthe common bus could be a synchronous or an asynchronous bus.

Still further variations are envisioned in which the CPU has sufficientlogic to allow the beginning and ending address of each main memoryboard to be set thereby allowing for the relaxation of the requirementthat all addresses must be contiguous (i.e., no holes in the main memoryaddress space). Further, variations are envisioned in which the systemwould dynamically determine, for example at system initialization time,which main memory locations are present and set the logic in the CPU sothat each memory address can be checked on each attempted memory access.

While the invention has been shown and described with reference to apreferred embodiment thereof, it will be understood by those skilled inthe art that the foregoing and other changes in form and detail may bemade therein without departing from the spirit and scope of theinvention.

What is claimed is:
 1. A data processing system comprising:A. a mainmemory capable of storing a plurality of words of data, each word ofsaid plurality of words of data having a unique memory address, whereinsaid main memory is comprised of a plurality of main memory units, eachmain memory unit of said plurality of main memory units capable ofstoring a unique set of words of said plurality of words of data, eachword of said plurality of words of data having a memory address that isunique from the memory address of any other word of said plurality ofwords of data in a particular one of said plurality of main memory unitsor any other one of said plurality of main memory units; B. a centralprocessing unit (CPU) capable of addressing words of data stored in saidmain memory; C. an input/output controller (IOC) capable of addressingwords of data stored in said main memory; D. a plurality of commonbuses, each bus of said plurality of common buses coupled to said CPU,said IOC coupled to a first bus of said plurality of common buses, saidmain memory coupled to a second bus of said plurality of common buses,said plurality of common buses for the bidirectional transfer ofinformation between units coupled to any bus of said plurality of commonbuses, wherein said information includes memory addresses, wherein eachbus of said plurality of common buses comprises a plurality of signallines, one line of said plurality of signal lines being an error line,and wherein said error line of said first bus is coupled to said errorline of said second bus via a connecting means in said CPU; E. a paritychecking means, included in said main memory and coupled to said errorline, for generating a parity error signal on said error line of saidplurality of common buses if the parity stored with a word of saidplurality of words of data does not equal the parity generated for saidword when said word is read from said main memory in response to saidCPU or said IOC making a read request; F. first means, included in saidCPU, for detecting an attempt by said IOC to address a word of data notphysically present in said main memory by comparing said memoryaddresses sent by said IOC to said main memory with the highest addressof a last word of data physically present in said main memory; and G. asecond means, included in said first means and coupled to said errorline, for generating a nonexistent memory error signal on said errorline of said plurality of common buses if said IOC attempts to address aword of data not physically present in said main memory, whereby saidparity error signal and said nonexistent memory error signal areinterpreted by said CPU or said IOC as an indication of a memory accesserror.
 2. A data processing system comprising:A. a main memory capableof storing a plurality of words of data, each word of said plurality ofwords of data having a unique memory address; B. a central processingunit (CPU), coupled to said main memory, capable of addressing words ofdata stored in said main memory; C. an input/output controller (IOC),coupled to said main memory and said CPU, capable of addressing words ofdata stored in said main memory; D. first means, included in said CPU,for detecting an attempt by said IOC to address a word of data notphysically present in said main memory; E. second means, included insaid first means, for detecting an attempt by said CPU during theexecution of a softwave instruction to address a word of data notphysically present in said main memory; F. address switch means, coupledto said first means, for indicating the highest address of a last wordof data physically present in said main memory; G. bus means, includedin said CPU and coupled to said IOC and said first means, for carrying aset of signals corresponding to said memory address from said IOC orsaid each memory address generated by said CPU; H. comparator means,included in said first means and coupled to said bus means, forcontinually comparing said set of signals with the indication of thehighest address of said last word of data physically present in saidmain memory from said address switch means; I. bistable means, having adata input, a clock input, a reset input and an output, included in saidfirst means and with the data input coupled to the output of saidcomparator means, for storing an indication of an attempt by said IOC orsaid CPU to address a word of data not physically present in said mainmemory; J. signal means, coupled to said output of said bistable meansand said IOC, for transmitting said indication of an attempt to addressa word of data not physically present in said main memory to said IOC;K. memory cycle indication means, coupled to said clock input of saidbistable means, for clocking said bistable means in a memory cycle whensaid bus means is carrying said set of signals corresponding to saidmemory address; L. first reset means, included in said first means andcoupled to said reset input of said bistable means, for resetting saidbistable means prior to the end of said memory cycle if said memoryaddress came from said IOC; M. interrupt means, coupled to said outputof said bistable means and said CPU, for interrupting the execution bysaid CPU of a softwave instruction if said bistable means stillindicates after the end of said memory cycle that an attempt to addressa word of data not physically present in said main memory has occurred;and N. second reset means, included in said first means and coupled tosaid reset input of said bistable means, for resetting said bistablemeans in response to the interruption by said interrupt means of theexecution of a softwave instruction by said CPU.